The PIC thing could be relatively simply decided.
From what I've read (I'm pleased to say I have no first-hand experience), if this was an instructional flight, i.e. the instructor was being paid to instruct, the FAA will pretty much inevitably consider him/her to be PIC. There have even been grey areas where a CFI was along for the ride but not being paid, and still held to be PIC on the basis of being the most qualified pilot. If the instructor isn't worried, he certainly SHOULD be.
Incidentally about the worst that can happen, as long as you file an ASRS, it really was an accident, and there was no real danger (no airliners getting RAs for example), is that you could be required to do an FAA checkride (I forget the jargon for this). All best avoided though, and a good reason for being VERY careful with Class B airspace.