The one major problem I have with FPAlpha's theory based on the rules established thus far in the story - never mind what's gone on in other media - is the "Guardian" having a green power ring available.
If they're holding off the reveal (based on other media) that the manhunters use the green tech as well, they'd better get to it, because it's getting to where any ground rules that haven't been laid down will feel like a cheat.
Other than that, the minor problem with the theory is that it's kind of obvious. But then, so was Zoe = manhunter, and they managed that when it turned out that the machine's identity won't be the major mystery of the show.
All in all, though, I like the theory. Everything about the Guardian's shady behavior fits into it nicely.
Here's the deal: the Stewarts don't know the exact date that Jordan got the ring, and it turns out there's a gap between that event and John Sr's later traumatic experience.