I have learned from previous games that doing "Propose State Visit" and "Commercial Agreement" have less than a 3% success rate and during the time they are in effect, there as not been an increase in your diplomatic rating. So it is a complete waste of time doing those diplomatic actions.
So this game, I only did "Request Expeditionary Forces." I was successful on 3 out of 119 attempts. Sort of. The logs said Bavaria, Wurttemberg, and Baden agreed to the request.
"2/142 This proposal, sent by to Baden, has been agreed upon by both parties: Request Expeditionary Forces"
However, even though the log said it was successful, no Wurttemberg or Baden forces were actually made available for French to use.
Unless you want to declare war, there really isn't any point to the diplomacy system. {Well, you could accept a surrender if you want to lose Victory Points, National Morale, territory, and send your country into turmoil.}