Henry, I have to agree with you about trapped solvents leading to cracking...especially the type described by the OP. Usually trapped solvents lead to the top coat, in this case the clear either delaminating after a period of time or popping (solvent pops) or hair checkered cracking ...ie wrinkling. This wrinkling (looking like cracks) would happen right after the clear was applied...not 6 months or a year later.
I also agree that what's under the single stage paint in all probability is the culprit, lacquer putty, filler to thick or to much hardner in it, wrong type of filler used like short strand fiberglass, mat fiberglass with to much resin and hardner...who knows until it's taken down to the source but, in my opinion there's something under the single stage paint that's causing the problems....to much time has expired for it to be top coating the singe stag.
Just my thoughts.