Well, without that battle Kenpachi wouldn't have been strong enough to take out Gremmy, who on his own could've ended the war for Yhwach, so...nah, she made the right choice.
You know, if the Anime does a 1000 Year Flashback when Yamamoto was Young with the other first 13 Captains, we can see the first Invasion the SternRitter of the Past did and see Unohana as the Kenpachi fight. That would redeem things because we all wanted that flashback, but never got it.
In her own words, she had to kill and revive him over and over again in order to bring out his full power and allow him to finally hear his Zanpakutō, which could only be done in a battle to the death. Hell, he didn't even INTEND to kill her and wanted to be able to fight her again, but she let him do it once he'd finally lost all his mental restraints because it was what she always wanted. So yeah, there's no "if she trained him well enough", this was the only way to prevent the war from being ended early by someone with the power to literally warp reality - even if she didn't know exactly that the enemy had such a combatant, she knew Kenpachi would be needed to take on the heaviest hitters, especially as a Special War Power.
Yeah I don’t buy that at all. Then again I think kubo jumped the shark in this arc. A lot of shit doesn’t make sense.
Hopefully the anime will extend and improve the final arc adding all the thing Kubo didn't have time to add in the manga.
Anime did it for Ashido, since he was supposed to be in the manga, but Kudo didn't have time and had to skip it.
You can not buy it all you want, it still happened, that's her canon motivation and explanation, it doesn't contradict anything about her previously established character and she's staying dead. In much the same way that, despite what I would PERSONALLY want for the story, Byakuya's still alive in a far more contrived situation.