Another follow up to the Schroedinger's Multiverse post but more of a story, in how-could-it-have-gone-s:
When someone time-travels in-universe, eg Hermione in Pride of Time - the laws of in-universe time-travel state, that she would have already been there the first time around, as nothing really changed, it was predetermined. So if the last breakpoint was during (not necessarily if prior to, as then the time-travel might just not happen) the time-traveled period, the time-traveled existence of in this case Hermione would already be sure in the universe, while technically originating in a point in time that has not yet happened...
Would then an inter-universal trip to that universe
a) Be impossible?
b) leave the original universe untouched and create a new universe in which the time-travel doesn't happen, despite Hermione's existence /prior/ to that point? Paradox, impossible.
c) leave the original universe untouched and create a new universe in which the time-travel does happen, despite other changes? This would require the person doing the time-travelling to be entirely unaffected prior to travelling back in time, as otherwise the paradox of the part of her past that is in the future being changed despite the part of her past that is already in the past being set in stone would still exist.
d) Change the universe while leaving the time-travelling intact- see above.
I don't like to think that it would be impossible, that in-universe time-travel is as good as a breakpoint, as that would mean missing out on a lot of stories, but it all seems to point to it being the case...
Well, let's ignore that for now, shall we?
Let's say Central travels to the universe, and actually reaches it, and we ignore everything above, even 'd)', and they arrive one after the other... Still of course not changing things in obvious ways, but acting 'behind the scenes'- saving Lily and James, and Mary and Marlene and many more, but leaving their bodied behind. Not letting anyone in on the secret of their survival until the time-circle is closed again. They do tell Hermione, anyone already in on her secret, too- and she is fast to join Central's cause, alone to pay back the debt of saving Mary.
Do they immediately show themselves in the open once the time circle is closed again? Or do they keep operating in the dark?
Does Hermione slot herself back into the timeline, or does she keep her aged body, more fitting to her husband, her child, her mind?
Even if they keep running black ops instead of going into the open, she could choose not to go back. Desiree and Delilah would probably even fight over the position... They would tell Ron and Harry, eventually- Delilah happy to have her friends back, afraid they won't like her as much as they liked the other Hermiome, Desiree harsh and ruthless in stating the truth, the whole truth, and nothing but the truth, of how universals are not the same people.
But really, what would be the bigger deception? A Hermione twenty years older than they thought, or one from a different universe?
Another idea: e) to kill that paradox, the universe would simply find the one way it /can/ fit the new predetermined way of the universe, now that a person (or several people in one go) arrived to it to the past... And if it can't, /then/ it is impossible to enter it. I still think it would be impossible most times, but well...
So Central would only send a group to enter the universe in one go, to be closed off afterwards until 19xx- either just that universe, or, if a split-off does happen, both. The group that foes manage to enter does what they can to save people, but they're at limited resources there...
But in the end, I think I do agree that it's impossible, so if I were to add this little adventure to the 'verse, Central would only be able to arrive once the first Hermione has gone to the past.
Unless
Unless unless unless
Someone's already there pre-timejump!!!
I'll have to figure out the logistics of then inviting in new people when they would come to the call of someone already there, but I suppose that might be too risky for central... Still, if they arrived, say, at the very beginning of the first voldemort war, done black ops and saved people and then noticed that they're caught in a time-circle... That way, they could have still saved everyone.
Either way, at some point or another, they would have told Hermione that she was right in trying not to change anything- not for the reasons she believed, you can't erase a timeline like that, but instead there would have been the danger of a new universe breaking off in which the change occurred while in the old one, the decision had never been made, only to perhaps being repeated later- leading to the danger of creating a splitter verse.
Except- can that even happen at all? Anyone from Central would certainly be wary of things that have the possibility of leading to a splitter verse, after all they had been the biggest catastrophes, tragedies, in Central history... But thinking about it properly, at the very least Cleo would realise that there can never be a breakoff point in the middle of an in-universe time-travel circle. The possibility of different solutions leading to two entirely different time-loops (or a universe without one) would have been pre-determined and the universe would have split prior to the loop. The existence of a universe /with/ a timeloop means that in this universe, it would have always gone in this one specific way, the person would have never actually changed anything, in this cause because Hermione would never have decided to do so.