Here's my take for what it's worth: The taxi driver from the end was actually Caine. He and Jack are "friends" or something like that, this makes a bit of sense because as you learn early on, Jack has been around for a very, very long time, so it's easy to see him meeting Caine at some point. I think Caine and Jack had their eye on you from the moment you became a kindred; they somehow saw your potential (I'm sure Caine could see this kind of thing) and decided to help you realize it. That would explain why Jack goes through the trouble of helping you out at the beginning of the game and why he hesitates talking politics with you (letting you make your own assumptions). Also, remember that while Jack did hang out with the anarchs, he said that he didn't really consider himself to be an anarch, just someone who believed in the same things the anarchs did, so again you wouldn't have your decisions swayed. Then right before you leave for your final cab ride, Jack tells you that it's time to make a choice about who you're going to side with, then Caine helps you to make a final choice while in the cab. I also believe that it was Caine sending you the mysterious e-mails, as like someone said before, Jack simply doesn't seem like the mysterious e-mailing type, while Caine definitly does.
Just my 7 cents