Hi PATTY; Sorry to hear of your troubles.
First thing I would do is get a good FLAT steel rule (you Brits have a ruler, but that's different ) and make sure the frets are flat to each other. By the variances you list in "cents off" it doesn't sound like they are. Individual frets can be re-set as needed.
You found a US nickel and dime on your side of the Pond?
Replacing the nut/bridge -- Like John, I've never in 40 years heard of a builder using plyuwood for the nut/bridge. It makes no structural sense. The countertop material that John mentions - Corian (tm) - makes good bridges. But so does hard, dry bone, purchased plastic bridges; and woods like hard maple, ebony, lignum vitae, or snakewood. Personally I find wood easiest to work with.
The usual spacing on the melody string couplet is about 3mm between the pair (1/8 inch to us Yanks).
Point of grammar... since the dulcimer is normally played flat on your lap, not vertically, most folks don't think of 'top' and 'bottom' strings the way guitarists or mando players do. We usually refer to the nearest string(s) as the melody string(s); next is the middle drone, followed by the bass or bass drone (farthest from you).
If you continue to have problems, you might get in touch with Robin Clark, up in Snowdonia, Wales at Birdrock Dulcimers.