Maybe he's not forgetting, just trying to double-bluff Hermione by appearing suspicious.
That's all true. And on re-reading, I'm going to go back to my original thought of Quirrel, with the emotional involvement being faked to keep Hermione's interest. Quirrel did, after all, speak up for her when all others were silent.
I agree. It was so obvious to me that Hermione was being Obliviated that when I read the instruction at the start of the next chapter I went back to see what I could be missing. It didn't occur to me that peope might not be getting it. And that was when it had ellipses.
bad memories are definitely scars you get from death
That's backwards. I suppose experiencing the death of someone close to you could leave a mental scar in the form of a bad memory. But that's hardly the definition of a bad memory. Nor is a painful memory an inevitable outcome of someone's death.
There is that. But it's just occurred to me: if the person attacking Hermione is not Snape, then what's Snape been doing to make him late and quite exhausted in Chapter 77?
Chapter 76: "And that's why I can destroy Dementors and you can't," said the boy. "Because I believe that the darkness can be broken."
This is interesting, because it touches upon a thought I had about the Dementors back in Chapter 45. In canon, Dementors are manifestations not of death or even fear, but of despair. (I believe Rowling has said she drew upon her own experiences of depression.) That's why chocolate helps, why they generate feelings of hopelessness, why they take away happy memories and leave unhappy ones, and why their ultimate power is to put people into a coma rather than to kill them. None of this makes sense for a manifestation of death.
But Harry's response would work either way. A happy memory, a pleasant thought, can shield against despair, but it can't destroy it. Hope, on the other hand, true grim hope – the belief that things can be made better and, crucially, the unshakeable determination to make them so, not by thinking 'wouldn't it be nice if…' but by knuckling down and solving the insoluble problem – is the only true cure for despair. And that sort of hope, which Harry shows, is actually pretty hard to hold truly, which would explain why almost no-one else has found the same way that Harry has.
I don't believe this was Eliezer's intention. Harry's views on death are far too close to Eliezer's own as shown in http://yudkowsky.net/singularity/simplified for me to feel that the Dementors are intended to be anything else. But I quite like my interpretation. Aside from anything else, since that sort of hope can be reinforced by being shared (enhancing both resolve and the feeling that the task is possible), it leaves open the possibility for a Patronus 3.0 by group-casting.
I have Sirius down as an outsider for Hat-and-Cloak.
Yes. I don't see Dumbledore in canon being so stupid as to fail to either ward his room against time-travel or recognise that that was how Harry got in.
April now, is it? Then the next thing that's going to happen is that everyone except Harry goes home for Easter (Easter Sunday was 19 April in 1992, and they'll probably take most of the holiday before it rather than after it since it's so late that year) and Harry's parents come to visit him. That should be interesting. I hope he's told them he's not allowed to leave Hogwarts.