Eleven: Folkcraft Resonator, 5 string Blue Lion Jam strung as a 3 string bass, Bonnie Caroll, Black Rose, McSpadden Kit [my first MD], six I made, including one made from Koa, and a travel MD only 4" wide, a Banjimer, and a TMB that I later modified into a double bass using bass guitar tuners and strings.
The Folkcraft, Blue Lion, travel, and the Koa are the ones I use the most.
It seem to me the 'when' needs a lot emphasis, like a larger text font, in color, no abbreviations and possibly other attention gathering techniques. It may help to include a link to a time conversion site. The 'when' is probably the most important piece of information I use to plan things.
Derik, you need to start at the third fret to get the scale you are expecting. The reason is the frets [excluding any half or plus frets] are laid out in mode patterns. In other words, each set of 8 consecutive frets [including the nut/0 fret] form a series of tones/semi tones [steps half steps]. This allows or supports. listing the eight notes in an octave to begin with each of the the notes in turn, eg., ABC#DEF#G [mixolydian pattern] or DEF#GABC# [ionian pattern], etc, each series being a different tone/semi tone order.
The most critical thing is getting their attention in the first place.
Dusty;
I kinda think you identified one of the primary sources of beginners in your original post, current dulcimer players interacting with potential players. It seems there is a need to motivate us to point new folks to appropriate 'sites. 
1. Glaringly obvious advertisement/schedule on a club home page.
2. I believe there is some way to boost internet search hits on the 'net.
3 Maybe Strumelia can come up with a way to advertise beginner classes, maybe a group or forum? I'm not sure the 'beginners group' would work very well because all posts move along in time, and sticky's [if made available] would eventually overload it. Maybe a new tab, 'Classes' at the top of the Home page, where the listing is for forums, members, rules, etc.? Inside could be all the different 'flavors: club, individuals. free, paid, beginners, advanced, n/d, finger,pick, flat pick, genres, etc.
Which brings up the existential question... if one were to put a fret marker on every fret- what purpose would it serve?
Could be both decorative and/or useful, depending on the design. 
I came to the dulcimer without any music background at all. Fret markers were just a decoration and my 1st dulcimer was a kit without markers. I didn't realize they had a purpose until I went to full chromatic fretboards. I looked at the 3-5-7-10 but they didn't make any sense to me [still don' t actually] so went to marking the diatonic frets since that layout was familiar. I suppose, considering they are really just location identifiers, that it's what a person gets used to using [without even thinking about them].
I just looked at my Folkcraft, it's marked at the 3-7-10. Didn't realize that before.
Inversion of Gm7 according to https://www.scales-chords.com/chord/piano/Bbgf
The difference is only a few thousandths of an inch. I suspect any difference in tone would be difficult to detect even with top notch equipment.
I've never seen one of the old ones. A call to Jim Woods at McSpadden may be helpful.
I installed one on a similar instrument. The one on the scroll was on the bottom of the scroll with the screw pointed more or less towards the nut. The bottom edge of the button was just above the scroll/bottom corner. There are 2 options for the rear one, on the side of the tailblock facing the listener, the other option is more difficult, re-position or share the anchors, and place the button between the anchors.
Where the wire hole is depends on the style of bar pickup. There are 2 styles I've used, one is aimed at violins with the wire exiting from the center of the bar, the other is the more common bar with the wire exiting from the end. Both are a bit longer than most fretboard widths, about 1 1/2 -1 5/8 I think. There may be shorter ones available. The end exit type can be carefully clipped shorter. The active elements are seen as small bumps in the skin, the cut S/B between the last 2 bumps, there 4. A good set of side cutters works. The hole on the end exit style is drilled right next to the fretboard and the wire/bar end covered with a lightly glued cap.
There are some that already have the bridge piece glued to the pickup element. I've not used any. I make the bridge to fit.
The wire can be fished out the jack hole with a piece of cutoff melody string with a hook bend on the end.
Use a pencil type soldering iron with a non corrosive, electronics flux core. Apply heat to the jack tabs to melt the solder not the wire.
Their site implies the files are pdf's. You may find them using a file management program [in the apps data folder maybe]. If so, you should be able to copy/move them to a new folder. You may be able to email them to yourself, they will probably be attachments.
What they said.
The wider fretboard gives more room between strings, if needed, a plus for big fingers and for 4 equidistant strings.
Both make excellent instruments, as do a bunch of other luthiers. I think it would serve you best to consider the mechanical features rather than 'models' or 'bling. VSL, fingerboard width and peghead styly [string replacement is generally easier on flat pegheads] will affect your intended use. I mostly fingerpick and have had standard McSpaddens and have their kit and have a Folkcraft resonator. I prefer, and really like, its short VSL and wide fingerboard. The McSpadden Ginger has a short Vsl also although I've not played one.
I learned several years ago that it is much easier to understand music theory, as applied to MD, when the word 'key' [in music] has several definitions. One defines or indicates the specific notes in a scale [the one you found], another, more generic, refers to the lowest note in a scale, regardless of the notes involved. The second one can also be a 'keynote' or 'scale center'.
So, in your tune, the written 'key' scale [def 1] is G; the 'key' on the instrument [def 2] is D.
The 6+ can be considered an additional fretboard overlay that modifies the mode layout of the frets. Two fret board layouts, 1 without the 6+, one without the 6.
Sounds good to me, the second piece especially. I think sometime is the better choice. Maybe a bass [one note on each beat?] arpeggio on some of the longer notes, or strum across all of the bass strings to finish. I also feel you will get much better at figuring out what works as you become more familiar with the instrument. You could also switch fretboards [octaves] for repeats or play octaves across 2 fretboards. Instrumental falsetto?
If you have a floating bridge it can be re-positioned, which can help some. Another trick is to make the base string VSL a bit longer by modding its slot in the bridge. That's done by refiling i so the string breaks over the slot further back. It may be too late to try this though since you've already lowered the bridge [many times, refilling the slot lowers the string].
Dime next to the 1st fret, nickel on the 7th fret. If its high on the 7th, then mod the bridge, then check both places. Be careful about modding the nut, a little bit can have drastic results and barely affects the 7th fret. I suggest doing the 7th first, leaving the space a bit high then doing the nut, if needed. If you have a zero fret, all change is done at the bridge since the nut there is basically a locator/separator.
A random thought occurred to me, how's your hearing? Hearing aids or not? Audio preferences? I quit messing with penny whistles and harmonicas because their sound is unpleasant to my wife's ears, too shrills. I have poor hearing so almost all MD's sound very similar to me, no nuances. And there's always the 'ego' factor.
Keep in mind being a 'luthier' can be a hobby or a business. If a hobby, the luthier can do what (s)he wants and can afford, In business, money and customer satisfaction is king.
I suppose you could use just about any D guitar tuning, or maybe the D pentatonic scale would work [DEF#AB]. There's a lot of room to experiment. Currently I can't see the center section being used with the 2 outer sections without a lot of playing around for awhile, probably finger/flat picking.
For string selection. You could go a couple of sizes larger since this tools recommendations are a bit light.
The curved nut/bridge could be replaced with straight ones if the wide fretboard is flat. Then you could do both a base and baritone or ???. I don't think using those as drones will work too good unless the MD sets are both in the same key. Even then it may be overkill.
The wide fretboard is basically a blank canvas that you could use to create something unique. Maybe a combined MD tuning like DADADA or DADDAA or???.
It looks to me like it is made to support a fretless bass guitar [or dobro?] and two different MD tunings. Bing Futch plays a double MD tuned that way. I would think any 2 tunings would work, although I would use a standard and a bass.
A quad MD! 
The 6 string fretboard could, possibly, be set up as 2 more, fretless [think chromatic], MD tunings. It looks like the spacing between the middle strings is a bit wider than the others.
A tip. Try not to cross your hands/hammers on alternate strokes. If you do a D scale on the treble bridge, for instance, start with the left on the D, which lets your right hand do the G [right side of the bridge] and the left hand can then do the A on the left side of the bridge.
I'm not a HD instructor but this is what I would do. Since you're probably looking at muscle memory instead of strength, I would practice with the weak hand only, then add the other hand occasionally to add coordination. Using your fingers to move the hammers should help as they move more accurately than your arms. Start by learning the finger motion one hand at a time until it feels more or less natural with both hands. Then alternate notes between hands as you move up/down a scale. Just do one note and alternate between hands if necessary. D[left ]-D[right] up/down the scale. Go slow at first to compare hands. Scales for awhile then a really easy tune like Mary had a little lamb, or whatever. Alternate hands/notes.
Does 'practice, practice, practice', sound familiar? 
I took an intense 3 day class many years ago. We did scales, a lot! Both on each side if the treble bridge and across both bridges. Rick also tried to get us to use our fingers to power the hammers instead of wrist/arm movement. That is done with the hammers held between the thumb and pads of the first 2[or 3] fingers [no death grip]. The thumb pushes down over the pointer finger while the middle/ring lift towards the palm. It's kind of a flicking movement.
You could try flipping the bridge over to eliminate the bridge [notch] as the problem. Hopefully the slant of the string will counter the bottom being square.
Have you checked the action, nickel/dime method? A strip of paper under the nut/bridge will raise the action.
Edit: In your user photo it looks like you're fingerpicking. From your hand position it looks like you may be slightly lifting the base string with your middle finger.
I guess y'all are discussin' that percission string support thingamajig. You know, the one on one other end from the string tightener doohickys. Back here in the hills, we call 'em the back string lifter and the one by the string tighteners is the front string lifter.
Don't suppose it really matters much what they're called as long as both folks doin' the discussin' are talkin' about the same part. 
Here's a few confusion factors just for fun;
Consider that pressing a string on a fret creates a new VSL, so fretting at the 7th [or other] fret leaves an unplucked portion of the starting VSL. Sympathetic/mechanical vibration could cause buzzing or unwanted [harmonic] overtones on the unplucked portion.
The ends of the string on each side of the designated/primary VSL form a sub-VSL and can be considered to produce a harmonic of the primary VSL.
The radii of the stop points are not being specified [just assumed] in association with the string properties, .020" R vs 2' R, one pretty sharp, one almost flat on an 1/8" bridge for example.
Etc. Etc. [Theory vs practicality vs experience vs opinion vs???]
Without precise conditions and measurements?????
What I hear [not very good] or do vs what someone else hears or does ends up being pretty subjective. My $95 kit sounded just as good to me as a classmates $500 custom MD.
Most all of it seems to work just fine.
I would just install the 4th string and use it. It will hold with no problem, It's pretty big and there is almost no leverage on it. If it really bothers you, cut it off, refinish and install the recommended pins. The escutcheon pins are standard for MCSpaddens, even the some of the older kits [I have one].
That shouldn't make a difference, I use a phone app on mine the same way and it is the same readings as the tuners I have. The slot for the melody string may be cut so the string touches on the tuner side of the nut instead of the bridge side, effectively making the distance to the 1st fret longer.
That makes it an interesting mechanical mystery if it has fretwire frets.
Use your tuner, then check the tuning at the 1st fret, if it's not close to what is expected, then probably just tuned. most all tuners available are set up default as equal temper. The space from the zero fret/nut to the 1st fret is usually too big.
Has anyone ever retuned a banjo into mountain dulcimer tuning?
Probably. I would imagine almost every tuning has been tried by somebody. An instrument with 4/5 tuners [or more] could be tuned [with string research] to just about any tuning using some, or all, of the tuners available. Would it be satisfactory? That would be up to the player. I play a 5 string Blue Lion with only 3 strings installed and it is great.
The name applied to the tuning probably depends on which instrument is being discussed.
