First of all, welcome to the doghouse...
I avoid taking chances with decals in the first place, since many of the kits I buy are 20 or even 30 years old.. I use a product called "Testor's Decal Bonder", which is a rattle-can of clear liquid decal film... It's simply sprayed on, then you let the decal sheet dry, cutting out the decals very close to the art-work.. If you just rough-cut the decals, you'll have a large area of clear film, so trim it tight..
This will thicken them a bit, but it also keeps them breaking or shattering/crumbling...
Another method, and one I prefer for belts/harnesses, is to make them myself from aluminum duct-tape. The metal tape will hold any shape it's given, and can also be left unpainted for the hardware.
As for your problem, once you add decal solvents to the mix, the problem is finished.. Unfortunately, so's the decal.. Unless you can slide it over the remains, that gap is there forever... I'd just make a new set... Seatbelt/harness decals look like decals in larger scales anyway, rather than seatbelts..
It takes some practice getting more than one set right, but it's still rather easily done.. Using tape will also allow you to pull and tug a bit on it in order to get it in just the right place...