Only thing I would say is the only real difference between the two is ambitions are always visible while plots aren't. It might be obvious to the person being advanced upon, but to everyone else, perhaps not. There's also the fact that plots can be forcefully dismissed by the player, but there is no such option for ambition. It's an interesting idea, but I think the whole seduction scene belongs as a plot, since it's less about romance and more about political gain.
What we could do is make an ambition equivalent that is more about romance than political gain, so it wouldn't have the same payoff (or the same risks) that the plot does but still accomplishes the task of getting whoever the desired person is in your bedchambers. There would probably be some limitations on this, to maintain the distinction between acceptable advances and advances that might piss one or more people off. These limitations would probably be unmarried people that don't currently have a lover, and maybe even only limiting it to count-level characters and below (I would think there would always be suspicion whenever a lower-rank person and the rich ruler just "happen" to fall in love).