Hello! Welcome to this site! We're happy to help you out with your questions about your McSpadden dulcimer.
Regarding the light scratches on your fretboard, you could use some liquid wax polish on the entire top surface, applied with some fine (000) steel wool, let dry then buff off the wax. This would also shine the frets at the same time!
The wood is black walnut, and everything but the top, bottom and sides should be solid wood all the way through. Light sanding should be OK. McSpadden didn't stain any parts-- it's all clear lacquer finish on the walnut.
Adding strap buttons is as easy as you say. I would center the tail button under the endpins. Drill a pilot hole for the screw in the tailblock, put a dab of soap or wax on the point of the screw, and attach the button. The top button can be attached to the heel of the peghead, either at the end or on the right side. This is a solid block of walnut.
These McSpadden dulcimers always had 4 strings. The six slots are there in the nut and bridge to allow for two different stringing configurations: In the equidistant string set-up, you set the first melody string in Slot 1, the bass string in Slot 6, and put the other two strings in Slots 3 and 5. In the double-melody set-up, you put the first and second melody strings in Slots 1 and 2, the bass string in Slot 6, and the last string in Slot 4. So you play the first 2 melody strings as one string.
Have fun!