Unless the only person that had the information is dead. There were theories a while back that Mab, Lea, or someone else might die before the casefiles are over. Others have speculated that he'll need to use Little Chicago to find something.
As for someone going back... the only reason to go back would be to help with the attack on Arctis Tor. But since the TTH theory largely calls for a stable loop via time bridge, the events occurring would occur once, and remain unchanged. If you throw that out, then it ruins the link to the events that already passed in PG.
For example, let's say Ferro decides to go back to help fight against AT. That means he doesn't recall participating the first time, and no-one that did reported his participation. But since we've already "seen" Future Harry's participation in events during PG, it means that whoever he followed back was also there.
The only way it works is if the person going back (or anyone they know, including surviving allies) doesn't know they already did, and that it failed.
But with players like Mab/Odin/Uriel, are there really bits of important information that could have died with someone? Maybe, but I can't imagine what it would be.
I was thinking broader than just PG for the time travel book anyway. Time travel is too convenient of a way to actually show all of the behind the scenes moves the BC has made throughout the series beyond just PG. It's a show vs tell issue from a writing perspective.
I don't think you have to throw out a stable loop with a chase book. Say some BC player goes back in time to try to change history. The right push at the right time can snowball into drastic changes. The BC/Outside has had so many close calls with big game over plots that you have quite a few chances to make a huge impact. Furthermore say that since time travel is so expensive to pull off only someone with a small enough metaphysical mass (Harry + possible sidekick) can go after them. It took Kringle to send the Wild Hunt forward less than a day (hours or minutes? I can't remember), so to send Harry years into the past may take a huge amount of power. (If the "rubber band" theory of time holds water, they might have to send Harry back several different instances to several different times)
At that point, Jim can handle it like Prisoner of Azkaban. Avoid your past self and try to change as little as possible, but the changes you do make still fit into the original timeline. The book becomes an investigation of where the BC member will try to change history and Harry somehow quietly foiling those different plans in the nick of time. So, Harry not acting would be an alteration of the original timeline and allowing the BC member to win.
PG basically requires time travel to make sense, so that will be a major part of the book. But you can also fulfill the WoJ of seeing Sue again by jumping to DB for instance. For a bonus if TT!Harry meets Mab at some point, she gets proof that one day he'll be her knight, hence her definitive statements about him eventually accepting. Really, we could revisit many of the major scenes in the series as a recap before heading into the BAT.
