I do wonder if it's the best way for the league to work. From what I've seen it means that the balance of power ends up being either so strongly for the Catholics that the war never gets declared, or so strongly for the Protestants that they steam role the Catholics.
One idea I had is that the league's could be broken down into two categories, "members" and "supporters". So only HRE members could be members of the leagues, but anyone can be a supporter of a league. Once the war starts it's only between the actual members of each league, however the supporters of each side provide some manpower and money, to the leader of the league they are supporting, each month. A supporter can cancel their support via the HRE screen at any point, so if it becomes to much of a drain they may well drop their support.
That way the war becomes much more about the actual HRE members themselves, instead of simply being decided by which way the likes of Russia, France and the Ottomans swing.