From Season 1 I've always twigged that Carolyn is either a member of The 12 or connected to them in some way that is not, "I work for MI6 and therefore must stop this nefarious criminal organization." There is just something about her that I've never trusted and I always wonder whenever Eve comes to her with a "Listen to what new information I've figured out about The 12!" if Carolyn is thinking, "Oh for pete's sake - took you long enough to figure that out." Like did Carolyn put Eve onto Villanelle/The 12 as a way to keep tabs on what MI5/6 knows/how close they are/next steps, etc. Just a theory I've had since series 1.
Eve automatically assuming that Villanelle was referring to them growing old together immediately took me back to Eve's conversation with Kenny in episode 1 when he asked if she was still "in touch with..." and she immediately assumed he was referring to Villanelle, not Niko. Girl's got it bad and always has. Does it make sense in the context of how Villanelle has "ruined her life", killed Bill, etc.? Of course not, but I love it.
The show has obviously had different writers and showrunners and gets renewed on a 1-2 off basis so I don't think there was ever any GRAND PLAN for a 4 series story but I wonder how much this seemingly romantic direction has been influenced by the sheer amount of chemistry that Oh and Comer have. I know that there are certain segments of the viewing audience that think it is simply "fan service" or "fan fiction" but for me the chemistry is absolutely undeniable. I am mesmerized when they share scenes together even if they are being total goofballs like at Paul's house.