I think it's safe to assume it's not the/a Q.
TNG Q would probably stop them anyway, seen as he's a big player in the Continuum and he'd not want them screwing with the Federation timeline.
Trelane is kinda non-canon.
Flint doesn't have the power or technology.
Kirk......No.
Annorax was 'reverted' and his technology didn't allow him to travel through time, just erase things from it. Though the Krenim could be another faction in the cold war.
It's too sneaky to be a Borg plot. But I suppose they could have allied with the Romulans, as they have in tons of books and Trek games and done it as a team.
And the Vorgons were too 'brute force' and their technology was too advanced. If they wanted to screw with the timeline they'd have just gone back in time and killed some people.
It has to be someone whose advanced enough to have the technology before the Federation have a means to detect it, but not so advanced as to be able to crush the Federation with brute force; And someone who's not part of a powerful empire, most likely a smaller race or a mercinary. If it is someone from a big empire they'd be using multiple agents in the past as opposed to just the one with the Suliban. Though if it's the Romulans or Cardassians, there's enough internal bickering for it to be the pet project of some nutter.
The best possible people/person for it to be would be the Ferengi, starting a temporal cold war just to make a quick buck.
