What we think we see in PG appear to be "nudges" done with great care.
This is a huge problem I have with the Harry time-travel theories as expressed.
First, <Watson Hat> Harry (or anyone else for that matter) going back in time for the purpose of preventing Harry from being injured is not nudging, it is direct action that changes the past. It would be the equivalent of the Gatekeeper going back in time to stop Molly from using black magic in the first place, rather than having Eb warn Harry that there is black magic being used in Chicago. It would be the equivalent of Harry saving the two girls at Splattercon! or going back in time to prevent Maggie Jr. from being kidnapped.
Second, <Doyle Hat> a story written for the exclusive purpose of explaining something in a prior book is lame. The explanation of what happened in the past book has to be relevant to the successful conclusion of the current book. Thus, no future book is going to tell the story of Harry going back in time to save himself, solely for the purpose of explaining the events of a past book.
There is a solution to both of these problems, fixing LC cannot be the purpose of any future time travel, it has to be something that happens incidental to the true goal of the time travel. The story would go as follows: in Book ##, Harry needs to accomplish task "x". In order to do so successfully, he needs LC, but as we all know, LC was destroyed in Changes. Thus, Harry goes back in time for the purpose of using LC to get the information he needs to accomplish task "x" in the future. He shows up at the apartment, heads down to the basement to use LC and sees a huge gaping flaw. He fixes it, chuckles when he realizes he just created and solved his own mystery, and proceeds to use LC for whatever it is he needed it for, then returns to the present time of Book ##. This also explains why Thomas is freaked out in PG (thanks Serack). Either Thomas is there when Harry shows up and finds future Harry disturbing or, Thomas thinks Harry is already home when PG Harry shows up at the door, and can't figure out how Harry snuck out of the basement without him hearing.
Now, here's my problem <Doyle Hat> with the above story solution: It's boring. JB could write a compelling story about Harry going back in time to use LC to accomplish future task "x" and completely freaking the crap out of Thomas, but throwing in - I also saved my own life - is boring and parallel to a major storyline in the Harrry Potter books.
The only solution that solves both the "no direct action" and "it's boring" problems is for LC to have been fixed by someone other than a time traveler.
2) As such, the only thing you can actually accomplish with time travel is something that already happened. If you kill someone in the past, then historically (your timeline), he must have been killed by you as well.
That argument is completely circular. How do we know future Harry can fix LC - because he already did. How do we know future Harry can't go back in time and make Molly his apprentice - because we already know he didn't. If LC was already fixed in the past, then there is no need for future Harry to time travel to fix it.
If Harry can go back in time and instruct Bob to mention to past Harry that LC was fixed for the purpose of causing Harry to go back in time to fix LC, then why can't future go back in time and instruct Bob to start up a discussion about that time he and Susan got dirty and start wondering if half-vamps can still get pregnant, causing Harry to give Susan a call and find out about Maggie Jr. long before the events of Changes. Under your theory, the only reason Harry can't do that is because he didn't do that, but the only reason we know he didn't do that is because JB chose to write it that way.