The theory is well thought out. This thread has also made me lol a good 10 times. Time travel is a big part of zombies but the one thing no one mention is the fact that the teleporters in Five are totally different from all the others:
1. They don't randomly teleport you to an area off map like Kinos i.e. Samanthas Room, Crazy Samanthas room, Meeting Room, etc.
2. They don't have a lightning flash, they have numbers all around them.
3. They don't require a cooldown time.
4. You don't have to intiate a link to another pad.
5. Kinos teleporter kills zombies and in five they can use them.
In conclusion, the teleporters in five may not even be powered by element 115 as there is no evidece of it. So if it is a completely different technology then the teleporters in Five may not be capable of time travel as there is no evidence of it. Also there are many different things around Five i.e. Thundergun and Ray gun. The Americans didn't make those, so they probably stole the teleporters from the Russias or Germans also. The Numbers were Russian but Steiner , a German scientist, help with the creation. So maybe the Thief is Russian or German, this would explain why and how he is breaking into the Pentagon. Hes a commie/nazi with a revenge issue. With that being said, my chronological order is a little screwy, because JFK is assassinated in 63 and Mason and Hudson barely figure out what is going on with the numbers in 68. So how can America steal tech that they don't even understand? With all that said It's quite possible that five is more of a nod to gamers by letting us play as these prominent political figures killing zombies at the Pentagon, and the Thief is a nod to Zork. But if I'm wrong and it is canon then I don't think Clarke is the thief because the teleporters in Five may not be capable of time travel (which is the only way to explain the aging), and in my opinion he physically (other than the markings) looks nothing like Clarke. It would make more sense if he was a different scientist working on the same project as Clarke, which would explain the equation markings. I'm not saying I'm 100% right, this is just my theory on the subject.
Sorry for such a big post. I articulated my thoughts as best I could.