Your peddling FUD. Just because you touched something in a way that wasn't specifically authorized by the original manufacturer of that something doesn't make you liable for anything that something does in perpetuity unless there is evidence that it's actually your fault.
Homeowners insurance covers the fire. They probably don't go after anyone unless they have specific reason to believe the loss was probably caused by a pattern of action that could result in a winnable lawsuit (good luck figuring that out when you have a melted lump of car to to work with). If someone does somehow go after the shop their insurance goes to bat for them and digs up all the reasons they're not liable and this was a reasonable repair that their customer performed.
Homeowners insurance covers the fire. They probably don't go after anyone unless they have specific reason to believe the loss was probably caused by a pattern of action that could result in a winnable lawsuit (good luck figuring that out when you have a melted lump of car to to work with). If someone does somehow go after the shop their insurance goes to bat for them and digs up all the reasons they're not liable and this was a reasonable repair that their customer performed.