DGR1 & 2 spoilers abound! Before we get started...
Some of the scenarios you list don't happen in the stories (at least DGR1 & 2,) so there is no way to know exactly how they would play out.
Further...
even if the rules regarding a certain scenario are well-defined, it's not as if Monokuma is obligated to follow his own rules 100% of the time (he usually does, but not always.) Since it has been shown that Monokuma does not need to follow his own rules, even if a rule was put in place, there's no guarantee that it would be acted upon as stated. In other words, any answer to "What if scenario X happened, if X never happened in the story?" is most accurately, "According to the rules, Y would happen, but since Monokuma can do whatever he wants, we can't know for sure what would really happen."
With that out of the way...
1.
Yes it does! (In a sense.) In DGR1 Chapter 4, Sakura kills herself by drinking poison. The verdict of the fourth class trial is that (despite physical assault from several of the other characters,) the murderer ultimately responsible for Sakura's death is Sakura herself, as judged by both the students and Monokuma. No punishment is given to the murderer, as the blackened is already dead.
2.
Doesn't matter, whether you intended to kill someone or not, you're still the murderer. In DGR2 Chapter 5, Chiaki unintentionally kills Nagito in Nagito's convoluted plot to reveal the traitor among the group of students. The verdict of the trial is that Chiaki, inadvertently or not, was the direct cause of Nagito's death, as ruled by the students and Monokuma. Chiaki is thus deemed the blackened, and is killed by Monokuma, as it usually goes. This case technically had witnesses and didn't at the same time. Whether that factor affects the ruling of the trial is purely speculation. My guess would be that it would not affect the ruling.
3.
No cases of unanimously agreed upon "self-defense" occur in the game, so this is one of those scenarios where we can't say for sure. However, two cases which were found not to be self-defense, but the potential excuse came up in the trial, were Leon's murder of Sayaka (DGR1 Ch. 1) and Teruteru's murder of Byakuya (DGR 2 Ch. 1). In both of those trials, there was no evidence that even if they were found to be instances of self-defense, that the culprit would be pardoned. My guess would be that even if the death of another student was a result of unanimously agreed upon self-defense, the killer would still become the blackened, and be punished if discovered.
4.
This doesn't happen in the story, so there's no way for us to know, but if it were to be ruled a suicide, then all the points in #1 apply.
However!
It's unlikely that this situation would come to pass, as Monokuma is shown to go to great lengths to protect the lives of individuals outside the scope of the trial and the victim/murderer setting, in accordance with the rules (you know, when he feels like it.) To be specific, Fuyuhiko nearly dies as a result of the events at the end of DGR2 Chapter 2, but, after being spurred on by the other students, goes out of his way to save Fuyuhiko's life, because Fuyuhiko was ruled not the blackened. So, my guess is that either any accidental self-inflicted death would either be proactively prevented by Monokuma, or ruled a suicide and tried as such.
5.
Monokuma is described somewhere in the games as "judge, jury, and executioner," meaning he is the one who ultimately decides the truth of any case, and he is also the one who enacts punishment.
At the end of DGR2 Chapter 2, Peko attempts to argue with Monokuma that despite the fact that she was the one who killed Mahiru, it was Fuyuhiko's will for her to do so, and thus Fuyuhiko was the "true" blackened. However, Monokuma rejects this argument, stating that Peko was indeed the killer, and that it is Monokuma's place, not any student's, to decide regardless.
6.
This doesn't exactly happen in the story, but there is a case close to it. At the of DGR2 Chapter 3 it is revealed that Mikan is responsible for a double homicide, but further that her personality was completely changed (or perhaps more accurately, "restored") after being afflicted by the "despair disease." She was correctly ruled the blackened, and received punishment the same as any of the other cases, so there is strong evidence to support the case that having a dissociative identity disorder or other personality shift is no excuse for murder, and the body containing the multiple personalities is punished wholesomely. Regarding Toko from DGR1, she was never involved in a murder, so we have no idea what would have became of her. My guess would be that regardless of what personality committed a murder, that person (i.e. the body) would be punished, same as usual.
