Hmm... Actually, I think this could be turned into a money-making venture!
YOU have all the original tracks, mixes, etc. and should have no problem proving it's your song (they certainly won't!), so... Since "they" have the claim, you should be able to find out exactly who "they" are and then sue them! Not just for "lost revenue" but for damages! Especially if they're tied to a music agency.
This could mean a lot of money for your lawyer you! ?