That's a terrible story. Five clutches and a transmission in under 80K miles. Just be forewarned, someone may post that maybe you don't know how to drive a manual. I assume that's not true, based on amateur psychology, and if you were the clutch killer you probably wouldn't risk posting about it on the internet
Some GTs had a problem with clutches, none as bad as yours though. What failed in the tranny that it had to be replaced? If the mechanic who says it's the axle is the one who replaced the seal and/or the clutch and/or the tranny, I'd be inclined to ask him why he failed to inspect the axles as part of any of those jobs.
I don't know if it's possible, but maybe he installed the seal wrong, or used the wrong seal. This cycle, did he pull the axle and can see damage/wear that makes a case for the axle being the problem?
There is a tendency to take it back to the same guy, to get him to fix his error for free, if it was something he did, , but if its been the same guy doing all this expensive transmission work on your car, given the totally unheard of frequency of problems you are having, it might be time to find another mechanic.
But I'm reluctant to offer a definitive course of action, really hard to say without knowing all sides of the full story.