I don't know about Sleeper Agent Michael, or any time travel shenanigans, but I've been saying since before the show aired (when we got the first spoilers) that the Romulans fit perfectly as the unknown, unseen antagonists. And I stand behind it, for several reasons:
1) They're spies. They have always been portrayed as underhanded tacticians, most especially in Enterprise when they used a holoship to get various races (Andorians, Tellarites, Vulcans) to fight each other.
2) They have cloaking technology. Disregarding Balance of Terror, we know they had some form of cloaking as early as 2152. Whoever pushed the Klingons into war (not that that wouldn't be hard to do) financed it by providing them with a cloak. A cloak that only they canonically had prior to that.
3) They hate the Federation, and most especially the Vulcans. The Klingon War was all about getting at the Federation for their peaceful expansionism. But the Federation last defeated the Romulans 100 years prior, and stopped all their plans aimed at controlling the region (and they seemed to isolate themselves, for some reason). The Vulcans are their ancient enemy, of course, and the Klingons have already attacked the Vulcans years prior to the War (Michael's backstory).
4) 10 years later, they have a technology exchange with the Klingons. Why would the Klingons and Romulans be so close in 2268, but not 2256. I mean, a number of reasons, really, but nothing says it's unlikely they were allies 10 years prior, and maybe longer than that.
In the second season, we're supposedly going to learn more about the Logic Extremists attacking from within Vulcan society. The first episode of Season 2, focuses on the Enterprise sending a distress call on the way to Vulcan. Vulcan has been infiltrated by Romulans before (V'Las in Enterprise), so it's highly likely they'd attempt it again.
I believe that the Romulan Star Empire provided T'Kuvma with the cloaking device. That they are, like in TNG, financing certain Klingon Houses to attack Vulcan and the Federation. That they have infiltrated key Vulcan posts, and are, in fact, behind the Logic Extremists.
Section 31 might also know all about this, and might be fighting a proxy war themselves against the Romulans/Tal Shiar. Michael might even know of the Romulan origins, presumably through Sarek, although Spock doesn't (unless we have to reconceptualize even more of his Balance of Terror lines...).
