I think pinched mantle is said to be from a parasite. I lost my last clam after it had pinched mantle and I didn't do the FW dip for the same reason you're hesitant, i.e., it was attached to a big rock. I tried toothbrushing it intead but it didn't help. Well eventually the clam died, so maybe it would have been better to find a way to do that FW dip. Was too bad too, that one was one of my longer living clams at almost two years.
I was going to say clams die way too friggin easy. But not with FW dips apparently. I remember a story of a guy who did a FW dip on a clam, then forgot about it, got distracted, etc. etc. and found the clam still in the FW bath the next morning, some 12 odd hours later. Put the clam back in the tank and it was fine. I imagine any parasite sitting on the outside giving it a pinched mantle was long gone!!!
PS. I wouldn't recommed a 12 hour FW dip though
