i WAGed that it would be used as a thaumaturgical springboard to travel back to chicago the way it used to be. like a little piece of history to use as the connection. but a duck nailed me with a woj that little chicago would be totally destroyed in a fire, though he never precisely says it was... its still great theory based on sound magical processes.
I haven't fleshed my "theory" out. It was more like stream of consciousness suposition. I had thought that Lash could potentially recreate LC, or any setting she was familiar with. I was honestly embarrassed to post that thought. I have seen LC posts degenerate into wild speculation, based on a very thin premise. I suppose I don't have the courage of my own convictions when I started speculating about LC being a potential device for time travel.
There are couple of really big IFS in my "theory"
IF#1: Time travel via mental projection. The OP supposes that Harry was able to project himself back in time into the role of his own subconcious. I like this theory, mostly because of the cool factor, but the dialogue with Harry sounds a lot like a more experienced Harry offering advice to his younger self. Accepting that as apremise I started speculating about how this might be accomplished.
Harry has projected himself outside his own body on a couple of occasions. Most recently when he was "mostly dead" and before when he used LC. It greatly resembled Astral Projection when he did it in WN. He placed himself in a deep trance (an unconscious state) and using LC as a focus was able to project his thoughts (his sub-conscious) into the city of Chicago. This was accomplished by using a scale model of Chicago (LC) in a meticulous recreation of its PRESENT state.
IF#2: Recreating LC in a past point in time would allow Harry to project his thoughts into said past. Ms. Duck pointed out the LC is a "puddle of goo". I speculated about what a person with an eidetic memory might be able to do and Neurovore pointed out that Harry may have Lash still in his head (she has an eidetic memory as exhibited in DB) and she can project detailed recreations of anything she has seen.
If#3: Lash is still in Harry's brain.
If Lash is still in there, she could recreate LC or a simulacrum thereof inside Harrys mind to use a focus. We saw Harry do a rather complex summoning ritual, without a focus object when he summoned Mab in GS. He did it all in his own mind. If Lash were to help Harry visualize Chicago, or any place else for that matter, at a certain point in time he would have the opportunity to project his sub-concious to that place. There is no place that Lash is more familiar with than Harry's brain, she has been there for years (maybe).
So,
IF all of this is true, Lash could at some future point create an image of Harry's state of mind via his own memories as it existed in FM and future Harry could project his sub-conscious back to that point in time to give past Harry a pep talk at a critical decision point in his life. This could allow him to affect the future and avoid many of the dangerous variables of time travel like the "butterfly efect" as the location of interaction would be his own mind. It also has the elegance of him not having to lie to himself. it really is his own sub-concious mind with whom he is having the conversation, just not his
present sub-conscious.
Whew. I'm not sure even I believe this but it seems to fit with a number of things we have been shown about the laws of magic. And it's cool! Also, Logistics is the guy who gets credit for this line of thinking. My ideas were based on his seed. He may not agree with what I have posted.