Someone else may have already come up with this interpretation, but I just wanted to get a prediction posted before the last chapter comes out.

Voldemort most likely had turned the diary of Roger Bacon into one of his horcruxes. The book was described as unusually durable, and it was given to Harry at a point in the story before Voldemort acquired the Resurrection Stone, so horcruxes would have worked only by touch. Needless to say, it was to his advantage to have one in Harry's possession, to be able to potentially take over the body of Tom Riddle v2.0 if the whole Quirrell thing didn't work out.

And now the diary is (supposedly) Hermione's horcrux too. Oh, dear. Since Voldemort wrote her into Harry's Vow as a required source of advice and restraint, he would have wanted to retain the option to influence that advice and force Harry away from dangerous decisions. So mixing horcruxes together in the same object might have been intended to give him the option to take over Hermione's body with access to her memories, if his plans failed to the extent of Harry surviving the night of the final exam.

With his personality now Obliviated, the mixing of souls might work in the other direction. My prediction for the final chapter is that Hermione has retained her personality but now has access, at some level, to Voldemort's accumulated magical lore, and possibly to things like passwords to his hidden caches of artifacts (since they'll eventually need to get the Resurrection Stone back to finally get the Hallows together).

This would be a neat last twist, opening a lot of possibilities for Harry and Hermione to kick off a collaboration on solving the problems of the universe in subsequent years. And the parallelism is elegant: the Girl-Who-Revived acquires a fragment of the vanquished Voldemort just as the Boy-Who-Lived originally did. We'll see if she gets a dark side too. ;-)

I think Harry spent the time sitting in front of the room planning what he was going to do to revive Hermione, because what went wrong when he got her killed was largely due to time constraints. It is explicitly stated that he was there for hours, and Minerva says it looks like years have passed when he comes out. So, I also think that whatever Harry planned, and then tried for several hours, did not work, and he came out with Hermione still dead. He is saying that there is nothing left to plan at the beginning of 92, whereas after he cools Hermione he thinks that he now has time to think. That strongly suggests he thought, tried the plan, and it failed.

On the other hand, Minerva has been told explicitly that people have generally not done everything they can, teaches Transfiguration, and quite definitely feels terrible over Hermione's death. She is also free to use the Time-Turner. So, yes, I also think she went back, Transfigured herself into Hermione, and let herself be killed, as that was, by that point, the only way to save Hermione's life. She probably borrowed Harry's invisibility cloak to hide Hermione from all the people who mustn't know that she is still alive if she is to survive. That hasn't happened yet, but there is still time. (Note that she also identifies with the other specified victim of the troll, Mrs Norris.)

This is well within Minerva's capabilities as stated, explains who was asking for the troll to be led away (Hermione under a cloak), and fits with Dumbledore's comment about losing another friend instead. It also has the potential to appease the feminists, because Hermione is saved by a woman acting heroically after Harry has failed.

I also find it a lot easier to live with. My reaction to death has got stronger as I've got older, and I cope a lot more easily with someone sacrificing themselves to save someone else than with someone just being killed. Agency matters. I still feel a bit sorry for the troll, though.