Hair line crack bottom back of walnut McSpadden
Instruments- discuss specific features, luthiers, instrument problems & questions
As Bob says -- it "ain't broke" so don't try to fix it. Especially since you don't know what Cyano Acrylate "superglue" is -- you'd be apt to glue your fingers together
! You need to ask McSpadden what they use for "varnish" -- it could be any number of things like polyurethane, varnish, shellac, various oils.
As Bob says too, those truly do not look like cracks, they look like grain going around a knot. Since you say there's no light coming through cracks, gluing would not help. Those marks could have been there since the beginning, and you're just noticing them. I don't even see any evidence of the "varnish" cracking there.
It NOT as if McSpadden, or anyone who builds dulcimers goes deliberately out of their way to hide imperfections, as your luthier suggests! That said, darn few of us builders can afford to just cut the back off and scrap the instrument or replace it. If there had been real cracks, McSpadden would not have sold it. Or they would have fixed the cracks and then sold it as a defect instrument.
Word of advice -- do not take your instrument to any old luthier for work unless absolutely necessary. Always talk to someone who is a dulcimer builder/luthier. Dulcimer are NOT guitars, they are not built the same way, they don't make sound the same way as guitars. mandolins and such like; and they are not 'set up' like those other instruments either.