Maybe they'd be justified if Cloud's emotional problems were a hanging issue from FF7. Or if CoT/AC actually showed him being unable to save others, losing his confidence and validating his slide into depression.
They were a hanging issue though. He may have accepted all that happened, but there's nothing that shows him dealing with it all. Not that I saw anyway. He barely has time to mourn for Zack and Aerith, his low self-esteem is still present and it isn't going to go away just because the game ends, and he still has to deal with the consequences of all the things that he went through which wasn't minor. There is a difference between accepting something and dealing with it. You can know something is true and still not have dealt with the emotional consequences of it. There is no time in game for Cloud to deal with his issues. The world is about to end, so he has to keep moving and deal with that element later assuming that he even survives.
But, no. His problems were completely contrived to repeat the same emotional journey. Cloud falls apart, his friends help put him back together, he regains his confidence, and everything resolves in a defiant one-on-one victory over his arch-nemesis. In the process mental weakness became his trademark and now Cloud is extremely unlikable to a lot of fans.
He falls apart because he gets buried in the emotional turmoil that came as the result of both the consquences of all he's been through and difficulties in coping. That's actually not an uncommon reaction to traumatic events. All in all, I think you can make a pretty strong argument that Cloud's suffering from a serious case of PTSD and that it's justified. It isn't like he cut his finger here. Not only did he start of with serious self-esteem issues, but he then watches his town burn, spends four years as a science experiment, watches his best friend die protecting him, has his own personality overtaken by a secondary one, watches another friend die after almost killing her himself, becomes convinced that he was constructed which leads him to give help Sephiroth with his plan (Giving him the Black Materia), etc. This is a lot to deal with and while I agree that he accepts it, I'd like to see where he has the time to deal with the emotional fallout of all this. There isn't time so he buries that part of it until the dust settles, at which time it all comes crashing down on him again. When the new stuff is added to it, he finds himself really struggling to deal with it all. He is missing five years of emotional development after all.
It's fiction. Literally anything can be justified. But just because it can be rationalized doesn't mean it makes for good storytelling. What if Cloud goes through the same process a third time? Or a fourth? Does it matter that it could happen to someone in real life?
That's not true that everything can be justified. Some things just don't make sense, but in this case Cloud's reaction is a reasonably realistic one and is one that many people fall subject to when they endure traumatic circumstances. Would it really have been more realistic for the creators to go "Issues? What issues? ... The game ended. All that stuff he went through? Well, that didn't really affect him. He's all better now!"
My issue is that it's not the same process, but a reasonable continuation based off of stuff set up by the original game. Cloud is not dealing with the same issues, but the fallout of the events of the game and the new stuff introduced like the prospect of his own death and that of another loved one (Denzel). That isn't the same.
Bah, I feel like I'm not explaining myself very well. I should stop trying. I'm sorry if I'm being unclear. Eventually I'll learn to keep my opinion to myself.
Edit: And it is different when there's a clear wrap up and when there isn't. The original game doesn't establish that Cloud's issues are all better, at least not that I could see. ACC concludes what the original began and from what little is seen of Cloud in DoC, that development held as it should following an ending which does show him having dealt with the issues instead of just accepting them.
As for your last point, I don't know. That would be up to you, I guess. I like when characters have realistic reactions rather then unbelievable ones (And, yes, I'd argue that Cloud's reaction is a very realistic one and it is one that happens.). One of my biggest pet peeves is assuming that the end of the game means that everything is all better, but I can't speak for anyone else. The fact that they tried to deal with Cloud's issues instead of handwave them away was something that I liked. But that's me.
I don't think that they could get away with doing it all again since ACC is largely about Cloud dealing with the issues and coming to terms with his past, which he does. They don't appear to be going that path. Cloud accepts all that happened in FF7, deals with the emotional fallout in ACC (as well as all the new), and then appears to have moved on by DoC. Seems like very reasonable, realistic development to me, but that's just an opinion.