Wow, I'm very sorry to see/hear of this incident. Had the car been recently refinished/painted? Paint thinner AKA Lacquer thinner shouldn't cause a problem. While it will eat through/remove lacquer based paint (the solvents allow the pigment to re-flow), modern paint systems to include your clear-coated car, are "2-K" AKA two-component systems that chemically harden. You can't undo the chemical reaction. Even if aftermarket paint, it is still catalyzed and shouldn't be affected. If anything was done recently to your vehicle; bring it back to that same body shop and they should stand by their work.
The problem is I have no idea what was used or what could cause this; it definitely looks like clear-coat failure, but doesn't look the same way clear-coat failure usually looks/forms.
Do you have any pics of the "dull spot" you noticed when you originally got the car back? I'm wondering if they potentially burned the paint / did something to cause de-lamination of the clear-coat from the base-coat, and whatever was used only helped the clear to separate even more.
I've worked on a lot of vehicles, and have seen a lot of paint up close and personal, but have never seen this before. I hope you can get things sorted out for the better of you, your pretty BMW, and the shop so everyone may move forward.