Further ado about 2 v 1 melody string(s)

I am both a beginner and this is my first post.  I apologize if this subject belongs elsewhere on this site.

Also, I do not wish to hijack the recent post on this subject, but I have a a couple of questions that have not been addressed in that post.

Background:  early this year I ordered a Clammer church model (presumably to be my one & only dulcimer) and it was delivered with a single melody string installed on the outside position on the nut & bridge.  It sounded fine to my ears, but I got to wondering why so many top dulcimer artists use two melody strings, so I called Clemmer’s shop and got a “you definitely don’t want to do that…you will not like the sound”.  Being hard headed, I put the second string on anyway and tuned it perfectly, according to my Zark tuner.  All I had to do was strum the strings once to learn what I was told on the phone – instrument sounded awful, as if it was way out of tune even though the Zark said otherwise.  I removed the second string, but it still does not sound right.  That led me to buy a used one to compare.

Since it is not practical to ship or take it back, 12hr drive each way, to have it checked out I decided to get a used McSpadden on eBay if the price was right.  Got a model M12W in like new condition.  It appears as if it was never played if not for a sloppy job on stringing.  It had 4 strings equally spaced.  I did not like the string arrangement, so removed them.  Have not restrung yet.

Based on the recent posts on 1 or 2 melody strings the consensus appears to be that a single melody string sounds better.  I am not at all disputing that majority, but I don’t understand the wide use of two melody strings by so many of the widely accomplished players, as I have observed dozens of times performing on YouTube videos or other venues.

Can someone clue me in?  What is the issue with single melody string installed outside vs in ?