Ms Duck, those are valid arguments... that don't really have a place in this thread. The reason is, ALL time travel plots tend to have those problems.
I feel like we're getting bogged down in trying to guess what the physics of time travel in the Dresdenverse is. Here's what we know:
1- There will very likely be a book that, in some way, deals with time travel (I believe the WoJ says something like, "Well, it's a rule, so you know Dresden is going to HAVE to break it in some way, right ? *wink*")
2- There are a number of obstacles/inconsistencies to overcome in order to fix Little Chicago. To wit:
a) Harry's threshold
b) Not be noticed/or mentioned later by Bob the Skull
c) Not be noticed/mentioned by Thomas
d) knowledge and ability to fix LC
Now, I think b) is actually the key to this. Why? Because Bob is loyal only to Harry. Bob will tell Harry anything EXCEPT if he can be convinced that that knowledge will definitely bring him harm to Harry (See Thomas's short story, "Backup," where Thomas tells Bob about the Oblivion War and swears him to secrecy).
Has it ever been confirmed that Bob can see through a veil? It seems like he would be able to- he can "see" magic.
My point is that, it would be extremely difficult and unlikely for a mortal wizard to fix Little Chicago without being noticed by Bob. One, because of the threshold- maintaining a veil while working on LC seems... foolhardy. Two, because no one, including Mab, knows Harry owns Bob. If someone, somehow, snuck into Harry's basement, they would feel no need to veil themselves (and again, it seems likely Bob could pierce a veil in any case).
So. With all that said, Time-Traveling Harry seems to be the most likely candidate. It's pointless to get bogged down in stuff like "Is Harry powerful or skilled enough to do that?" because it doesn't matter- there are any number of plot devices Butcher could trot out to lend aid to Harry in those departments. Uriel, Mab, the Gatekeeper, Odin, and more.
However, there isn't much Butcher can do about the facts I listed above. Any solution will have to have some explanation for them.
So, logically, that leaves a bunch of superpowers (Mab, Odin, Etc)... or a time-traveling Harry. Of said superpowers, it'd pretty much only make sense for Mab at that point in the story.
So, as it always seems these days, we're left with Mab or Time-Travelling Harry.
(I'm assuming here that if anyone other than TTH did it, it would NOT be through time-travelling. Mostly because that would be kind of needlessly convoluted. Imagine reading the last book in the series, and suddenly Mab says, "OK, before we go fight Outsiders, I'm just going to go back in time without you and fix Little Chicago." Not gonna happen. If time-travelling to fix LC happens, Harry will be involved.)
When confronted with the choice between Mab or TTH, I side with TTH... mostly because it would make for a more interesting story. Would it really be that interesting or shocking if, in a few books, Mab says "Fool wizard, of course it was I who repaired your focal device. I deemed it... neccessary." No. We the readers would just go, "Oh, I can kinda see how that makes sense," and then we'd continue on with the book.
However, if Harry was on some kind of epic time-travelling quest that spiraled through his history, and he was in the PG-era and had to find someone RIGHT NOW... it would be much cooler if he opened his door, talked his way past Thomas, went downstairs, fooled Bob (or, more likely, talked to him and then swore him to silence), repaired LC (which he could do since he already knew what was wrong), used it, and oh, by the way, took one of his reference books and put an old "Calvin and Hobbes" anthology in the gap where it would not be discovered for another year!
tl;dr Only Mab or TTH could have done it, and TTH would make for a much cooler story.
