Maybe instead a large bonus modifier instead of removing negative attitude modifiers. I don't think a good ambassador can make a nation forget about problems, but they can make other nations more friendly and forgiving. I'd also say that instead of requiring it to be in the capital, perhaps it could give higher bonuses for the territory type, with foreign core giving the fewest bonuses, core giving the highest, and colonies halving the bonus while capital overriding with the highest possible. This way it's best to do it in the capital, but you still have the choice of doing it elsewhere. Perhaps the bonus could also spread in smaller amounts to allies of the target nation, allowing players to mend small hiccups with multiple nations in a single bound. I can imagine this would be useful when you have a bunch of nations teetering just off friendly, and one that's friendly with all of them teetering just off annoyed. You could fix your issue with the annoyed civ and top off onto friendly with the others in one go.