Michael Gray & Gina Gray
Why it might be him/her: Few characters this season have inspired as many theories as Michael's new American wife, Gina. That's largely due to how little is known about her background and how pointedly antagonistic she is to Tommy, Polly and others from virtually the second she meets them. Not helping matters: Michael, who had been handling the Shelby investments and business affairs overseas, married her on the voyage to Birmingham, allowing Gina (and writer Steven Knight) a convenient means of cloaking her maiden name. Could it be... Changretta? Doubtful -- even Michael would have enough sense to steer clear of a Changretta. but it wouldn't exactly be surprising if a big reveal about Gina's background were on the agenda for Season 6; maybe she's a long-con-playing, vengeance-seeking daughter of Adrien Brody's tiltingly behatted Luca Changretta using an alias or somehow affiliated with the Spinietta crime family. More likely, she's working with Mosley in some capacity, given how attentive she always seems to be during Shelby family meetings, and how, at Tommy and Lizzie's party, she already knows that Mosley is a baronet when Michael points him out -- and how she later exchanges a knowing look with Mosley during the weird ballet performance. (Many fans have theorized that she is also the mostly unseen woman Mosley has sex with in his dressing room before his speech, but that makes zero sense logistically or logically.) One final point in favor of Gina being involved with thwarting Tommy's assassination plot: her final speaking line of Season 5, right after Tommy rejects their proposal and tells Michael and Gina to leave, is "Guess we're going to be forced to take the second option."
Why it might not be him/her: It's too obvious. Michael is clearly rising against Tommy but seems to prefer doing it by the "rules," or he wouldn't have put together a prospectus. It's possible he's in league with the IRA -- remember the phone call Tommy received from a Northern Irish operative who claims Michael was scheming against the Shelbys. But there's not much else besides that call to go on (although perhaps the IRA is Gina's "second option"). The few details that have trickled out about Gina, like that she has a New York-based family who is very experienced in distributing drugs, and her influencing of Michael to blow up his relationship with Tommy, makes me certain that something is up with her. (Many fans have also posited that Gina may be lying about being pregnant, but there's no verifiable proof of this and I can't see how that would make much strategic sense for her in the long term.) Whether it all adds up to thwarting the Mosley assassination is another thing, as Gina wouldn't have known where the sniper was, either, since that conversation took place after she and Michael had left the chaotic family meeting where they made their proposal to restructure/take over the Shelby organization.
Verdict: Red herring. I will say that Michael and Gina likely have something to do with undermining Tommy, but I'd also bet that "the second option" doesn't actually refer to what went down at the Mosley rally. I'm also not convinced that Michael knows the entirety of what Gina's up to, if she is, in fact, up to something, and that he might not be too happy when he does find out her full agenda.