I've used the hex head vent liner since I got my rifle and have not had the same problem your describing. I do put some anti seize on the threads when I put it back in the rifle, just a big dot of it with a tooth pick. Also I never try and remove it until I have pumped some hot soap and water through the barrel first. I figure that helps break any seizing that might have taken place.

As for removal of the vent liner, if it is cracked, I would take it to a gun smith. That or maybe a would a tap work to get it out, then you could replace it. The big thing is you do not want to hurt the threads there...

Good luck, and calm down. It will all come out in time.