About Yotsuyu:
I think that at least Maxima had believed that he was truly going there to make peace with Doma. Asahi needed Yotsuyu for destroying the plan because she was seen as someone from Doma thanks to her memory loss. Thus her standing there and talking about how she will rule Doma and then turning into a primal can be seen as a break of the contract because she was still seen as part of that country. Thankfully Asahi died and Maxima turned out to be truly someone from the other faction and someone that has a brain and saw that this was a set up to break peace again. In the end Yotsuyu was the most important part in the plan because if she had not turned then Asahi would not have anything that shows a break in contract. So in that way if his plan had gotten through Yotsuyu would have been partly responsible for another war and tension for Doma.
Also I still believe that this was Yotsuyu. I mean Tsuyu with no memories was like a child. Would a child really be able to fool everyone around her and then break out again to kill herself? Would the innocent Tsuyu really be able to act in her old ways? For me this was a sign that Yotsuyu has returned but as the primal shows she has a inner conflict. Because maybe for the first time in her life she (or more Tsuyu) was loved. Maybe that just completely broke her because that would mean that she finally had to face the consequences of her horrible actions. Maybe she cant just tell herself that her behaviour is justified? Thus her trying to kill herself.
I believe that she is quite aware that as soon as anyone sees that her memories are back she would get killed or sent back to Garlemald (and who knows what they would do to her). She was only alive because she had no memories and thus they did not want to judge her. Having them back makes her also able to be jugded and no matter if she behaved childlike for a time or maybe regret it a bit, she still did horrible stuff which would probably end in a death sentence. And with her memories back none of the people in Doma would probably even want her around.
In the end for me this was the best outcome. It may give Hien the motivation to make Doma in a better place (but realistically bad stuff will always happen, that just how it is), Gosetsu will go around and atone for the stuff he did under command and the people are finally able to heal their wounds without fearing that her memories might come back and thus they get punished again. And for Yotsuyu, well it might sound a bit strange but this might have been the best outcome for her too. She may have had a chance to live as Tsuyu but I am not sure if that would have been something forever. Where would she live? Would she be confined to one place her whole life because people would maybe just take their revenge out on her? Or maybe need to stay with guards thanks to the possibility of getting her memories back. Gosetsu is also old and wont always be around to protect her. Tsuyu was her only chance but I am honestly not sure if this was a great life either.
As Yotsuyu herself she would have either be killed or land in prison for the rest of her life where she will have to face the stuff she had done. There was no realistic way that she would have been able to go outside. So her being able to take down these three people and then being death while still at least having an understanding on what she did was probably the perfect way to end her arc. And its also a good way where I can accept the last few patches about her.