I use proxy wars a lot. Though the risk of an early peace is always there, the important thing is to distract alliance members you don't completely trust. A Castilian-English alliance aimed at France also works well.
I once used an almost identical proxy set-up, only Sweden wasn't in the alliance. Poland was and I used it to slow down Austrian expansion. It was a disaster, sadly. Poland got thrashed mightily and Austria got military access. General Winter saved me many a battle. Until Sweden joined the Austrian alliance.
I once used an almost identical proxy set-up, only Sweden wasn't in the alliance. Poland was and I used it to slow down Austrian expansion. It was a disaster, sadly. Poland got thrashed mightily and Austria got military access. General Winter saved me many a battle. Until Sweden joined the Austrian alliance.