The thing with the Romulan senator was the first actually successful piece of Federation-Romulan cultural outreach in history. My headcanon is that the Tal Shiar pretty much immediately figured out what actually happened but were so touched by the effort to conduct relations in a culturally Romulan way that they recommended the alliance anyway.

False flag assassination and planting evidence is basically the Romulan "Darmok and Jalad at Tenagra"