Wasn't that more fiddling with his perception of time rather than anything else?
Yeah, I think she dosed his brain with tachyodorphins to make his brain work super-fast. But she also says at the time (pun!) that time doesn't exist.
WN - pg403
“Time does not exist,” she said, her tone firm. “Not the way you consider it, at any rate. I have temporarily accelerated the processes of your mind.”
And that was what she was doing to accelerate the process of his mind in the physical world. As a non-corporeal entity, she might be able to show him the past just as easily as the present. I think that's why she has eidetic memory: everything that existed in her past exists in her present, so recall isn't a problem.
The problems would arise that she couldn't manipulate the past, as that's probably a no-no, and she probably couldn't send Harry back prior to her seeing/experiencing everything in his mind.
The work-around for the second is that, as part of Harry and sharing his soul, she has his memories as well, and might be able to go back to that memory.
The first isn't as easy. But its assuming that such things are off limits to Angels and Fallen. Maybe by sending Harry back, but not herself, it wouldn't be a violation.
Harry Lash-Back Theory (sub-category of logistics' Time-Travel Theory)1) Harry finds out in #15 that the parasite is Lash, who is sharing/living/feeding off of his soul.
2) They resume 2-way communications.
3) Harry has a need to go back and revisit his memories of previous casefiles.
4) Lash reveals that Angels and Fallen are non-linear, meaning they can exist and revisit any time from their existence.
5) Harry realizes that as an aggelocopy feeding-off-of/sharing his soul, Lash can exist and revisit any time from his existence as well.
6) Lash tells him its against the rules for a shadow or Fallen to go back and change the past, and that she can't visit his past.
7) Harry says that
she wouldn't have to revist his past; only he would; and he promises not to change anything.
8) Lash shows him how to not only recall his past, but to actually revisit it, in a non-linear mental state.
9) Harry visits himself to piece together a few facts, and takes the opportunity to talk to himself, knowing that he can only have the same conversations over that he had previously. Hence his deeper understanding of events, as well as his reluctant knowledge of not being allowed to change things.
10) Harry then returns to the present.
Problems with this Theory:1) Making a lot of assumptions on Lash's abilities.
2) We still have a paradox, as with all time-travel scenarios, because Harry has to recall a conversation he already had with his future self before his future self existed to have the conversation, meaning that the future self would have had to have existed in a timeline where a future-future self didn't make the trip back, which means that that first conversation would have changed the future. :o