Sorry, I didn't--and still don't--completely understand what you're asking. I'm not trying to be an ass, but I think you're asking a few questions for a friend or multiple friends and then one or two for yourself. I'm tryng to reconcile where the question about TPIC fits in with the GoJet questions as you don't sound like you would consider working there.
I have a special place in my heart for GoJet--at least the group of pilots that left Trans States when GoJet was started. They collectively and individually stabbed an entire pilot group in the back for exceptionally selfish reasons. I wouldn't call them scabs, but they're not far from it in my book. I know many people have since gone to work there who had nothing to do with the original group. Some were even furloughed from United, et al when they began working there. I wish them no ill will and my previous post belied that sentiment and for that I apologize to them, and you.
Anyway, in the interest of being helpful and as a mea culpa for being admittedly crass in my previous post, I'll attempt to answer your questions.
1.) In spite of my personal bias against GoJet, I don't think that majors look poorly upon someone that works at GoJet simply because they work there. I think they're all looking at the whole package and where you work is just part of that package. Certainly other airlines have better reputations for better training, etc. But I don't know if that is affecting hiring THAT much. Certainly their pilots are getting hired at majors. I think some majors (JetBlue?) posts lists of candidates in the pilot lounge and current pilots are welcome to recommend NOT hiring problem children. I would think that any Waterskiers that were around Trans States circa 2005-06 would likely speak up if they saw any of the original G7 clowns that circumvented their seniority when they left Trans States for G7. I believe most of those guys are still at G7--either by coincidence or because of this fact. Doubt it's by choice.
2.) I would think that a termination for that reason would be pretty insurmountable in nearly any hiring environment. I know a lot of the FSI guys and I know how their corporate relationships with airlines work. I'm not calling your friend a liar, but I can't imagine a scenario where a third party instructor would lie to get someone fired for an offense as agregious as threatening them. Again, not saying it didn't or couldn't happen, but if I'm skeptical, imagine how skeptical a hiring board would be about this type of thing. If it DID happen, it really speaks to the stability of the candidate and becomes a liability issue that I can't imagine a major airline would care to undertake.
Regarding potential legal action, I would think that the Teamsters would have grieved his firing. If he was a probationary employee in training they may not have--although most ALPA shops typically grieve even probationary firings unless there is some serious cause in the firing. I'm not sure of the practice at Teamsters, but if that's something they routinely do then the fact that they may not have in this case is probably quite telling.
3.) If you're asking what I think you are, I'm pretty sure the majors know that certain regionals have reputations for a ready, shoot, aim approach to firings. That being said, I do not think that that would in any way salvage the prospects of the individual in the previous question.
4.) The TPIC thing has been talked about at length on here. Seems it's less and less important. If you're at a 121 carrier, I wouldn't leave the 121 world for TPIC. If you're not already doing so, I'd concentrate on the other things that seem to be getting people hired. Training/Standards work, volunteering, union stuff, etc.
I AM pretty fun to fly with. When I was a captain, I'd even let the FOs fly at least one leg almost every trip
Best of luck.