I just realized how long this was, I apologize in advance
ok, I gotta admit I haven't actually finished the series, the first time I read through it I only got halfway through book 40 (long story) I've been reading the ebooks over the last couple months and just read 41 a few days ago, I definitely could have done without that one, other than the sheer weirdness of it, after the first 15 pages or so I felt like reaching in and strangling Jake, but I digress. My point being I can't really comment on what happened after that, so I'll have to concede whatever happened in 43 and mm4 for now, if anything happened there that invalidates what I'm about to say, just tell me that rather than going into detail, I'll trust you and eventually read through them.
As far as an ability to handle stress, that's a complicated issue. It actually has very little do with personality, none really. Well not in susceptibility to stress issues, although it does in how quickly you recover from them.
As to what does affect susceptibility to it I'll focus on the ones relevant to Tobias
Israeli studies found that the strongest factor which distinguished between soldiers who were decorated for heroic acts and those who became battle shock casualties was having had many recent changes on the home front.
I think permanently becoming a hawk qualifies, as does finding out what's her name was really v3
The second strongest factor found in the Israeli studies was unit cohesion: the soldiers who became stress casualties were often committed to battle alongside strangers, while those who became heroes were alongside unit members they knew well, trusted, and depended on.
Since Tobias was the only member who had no prior connection to the members of the team whereas the other were friends or related he was the only one who had this risk factor
Of course it's a moot point if they continue to influence him, but as time goes by he should accept that he's become a hawk, mostly removing this risk factor, and would learn to trust the team members as they fought together more, so these two only really influenced him for his breakdowns in 3 and 23. Although I can't really recall any serious problems he had other than those, at least prior to book 42
I'm noticing a definite foreshadowing that Tobias is about to have a breakdown, although I can't figure out why Jake never takes notice of these things and try to fix them before they become problems
Anyone may become a battle fatigue casualty if too many high-risk factors occur
At several points every other member of the team had similar breakdowns. Jake in 8, 11, 31 and 41, Marco in 15, 30, and 35, Cassie in 9, 16, 17, 19, mm3, and 41, Ax in 8, 18, 26, and mm3, Rachel in (maybe, I can't quite remember) 22, 33, 37, and 41.
So I'm not really sure that he couldn't handle the stress, at least not worse than any of the others
I think Tobias wouldn't be able to lead because he wouldn't have the confidence or command presence to do it. Given some time and maybe a little guidance he would be able to, but I think the same holds true for most of the others as well. In the long term, I think Tobias would probably be a good second choice
@kotetsu
I agree that she never really got them any good morphs except maybe the birds, I can't remember how they got them, but she did know which animals to get and how they'd react, like knowing to use the squid to get to the ship and whales to get the squids