A lot depends on the specific story being told. For example; and I'm just making this up on the fly, it's not an idea or scenario that merits serious discussion, if at the end of the series we find that Harry = the original Merlin, and we find that Harry has learned new complex kinds of magic that Bob isn't familiar with, and he goes back in time to build Demonreach and do the other stuff Merlin did like start the White Council, then nothing has really changed except our perception of Harry, plus it's also how the story ends and how Harry leaves the stage.
If Harry travels back to a previous story like Proven Guilty and nothing changes, or perhaps Harry prevents another time traveller from making changes to the timeline, there could still be a major impact on the rest of the series from what Harry learns from his time travel adventure. It doesn't matter if Mab = Murphy or Molly, it could be something else of major import.
There is also a third possibility. There's an episode of the old Twilight Zone where through some unexplained process a character goes back in time to the night when Abraham Lincoln was killed and he tries to prevent the assassination from occurring. He fails of course, but does change one small detail in the timeline. A police officer who believes the time traveller tries to warn his superiors that the President needs protection and even though he fails his diligence is remembered which leads to his promotion and eventually, we are told, he became Chief of Police and with his higher salary and probably with some advice from upper class contacts, he made some investments which made him a millionaire. At the end of the story we see a character who was just a waiter serving a bunch of wealthy men discussing time travel at the beginning of the story, is now one of the wealthy men telling them how his family came into its fortune. So Jim could do something similar. He could send Harry back in time and have all the major events of the original story happen just the way they did before, but TT Harry has a conversation with someone, and through that conversation he alters the future in some subtle but important way.