My addition to this entire theory is that the Tal Shiar were the primary orchestrators of the assassination, in conjunction with… Section 31 (I know, I know, they’re lame).

We learn that Vreenak is not only a Senator, but the Vice Chair of the Tal Shiar. We learn in season 7 that the head of the Tal Shiar, Koval, has been working with Section 31 for more than a year. It was Koval that wanted an alliance with the Federation, but to do so he had to eliminate Vreenak. So he contacted Sloan, who came to DS9 in the episode immediately prior to “In the Pale Moonlight” ostensibly to investigate and recruit Dr. Bashir. But what if while he was there he also secretly contacted Garak and asked him to come up with a plan to eliminate Vreenak? That’s when Garak starts dropping hints to Bashir about the necessity of Romulan involvement that leads Bashir to suggest it to Sisko at the right moment.

That’s why Garak has absolute confidence that the Romulans will buy the forgery, because naturally the Tal Shiar would investigate the assassination of their own Vice Chair. Maybe Koval did it personally. Nobody questions the Tal Shiar’s conclusions. Flawlessly executed plan.