1. Yes, I (and the law) still call laws of passion murder ("crimes of passion" are not the exact equivalent of an "insanity plea.") They are considered murders of a lesser degree, but still murders along with a prison sentence. The difference is that it is not a first degree murder. I would also think killing others really can't be comparable to suicide. At all. But that legally, psychologically, and mentally the only big differences between the two is that more people have a chance of noticing and helping someone who has shown symptoms for a while compared to someone who does something spur of the moment. They're not usually a class of people who wouldn't have attempted "except for this one thing that happened that made me crazy." Certainly, we know this isn't the case of Tobias, who was addicted to morphing, became a nothlit, and then spent his time living bitterly going on about how he'd never be human again. Tobias fits the case of what, 25% of people who attempt suicide? (By which I mean, not showing symptoms that make people concerned he's suicidal and probably not planning anything or contemplating it - not that he's a hawk.) He's a ticking bomb.
I would also argue he's far from long-term peace in #23. It was more like a temporary peace (which seems to be something he finds at the end of all of his books, only to have it end up being irrelevant in the next). In the next book (from 23) he narrates, he experiences all those same internal conflicts when he gets tortured by Taylor, so that conflict still lived in him somewhere. Then again these sorts of problems - his next book also involves Taylor. Then he finds his mother and he goes through the same struggle as he tries to figure out why his mother didn't care for him. (And, for crying out loud, she's blind with amnesia to where she didn't even know the definition of basic vocabulary and he wants to tell her she should have cared for him?) We often decide he's happier as a hawk, but the truth is, if we go based on narration Tobias is a lot of consistent internal struggle.
2. To the morph thing - we know Arbron never lost control of his morph beyond the usual Taxxon need to eat, which is such a heavy drive even a Yeerk can't completely suppress it. However, when they've lost control of their morph, they are not aware of themselves. That seems to be a requirement in all morphs except for those completely overrun by some instinct. Ant, termite, Taxxon. You don't lose control while completely aware of who you are - it's being aware of who you are that seems to determine whether or not you'll be able to control the morph (same reason the Hork-Bajir were generally not considered good for receiving morphing powers by the Animorphs). Tobias, as he puts it repeatedly in the series, is his hawk morph. He didn't have some long-term, completely conscious blip in the ability to control his hawk morph. He just decided it wasn't worth controlling the actions. He's talking about a different type of tired than most people are ever even aware of. And I don't know, but I guess in my opinion trying to deny that side of him just degenerates his character, or part of what his character is about. He's supposed to be the kid who takes life too seriously, can't hold out for better times, etc. Tobias is, essentially, the kid who lets their current life situation dictate their future. I often argue the point that Tobias is sort of like the person who commits suicide from the beginning - he's gone from the human world and he gets to see that people didn't even notice, or that they just moved on.
His quote from MM#4 is an attribute to that part of his character - when he talks about how his life would have sucked, but he could have waited. High school wasn't forever, living with his aunt and uncle weren't forever. But that he'd looked for some salvation that just wasn't real. ("Giving up some of your freedom" probably isn't something anyone should ever agree to, even if they assume it's metaphorical - comes off as a bit cult-like, really.) It's a shame he couldn't have kept some of that internal revelation in the regular timeline, it might have helped him a little post war.