In addition to all of the above, I could see such a thing happening in Britain, actually.
Scenario:
Glorious Revolution does less well, and when the Whigs fall from power in Parliament, the Tories invite a sane Jacobite pretender (note that no such person existed iotl, and if he had, William might never have gained the throne in the first place, still.)
There's a minor civil war, but the Tories win ('cause no one likes *Willliam very much anyway). Then the Jacobite pretender blots his copybook, and when the Whigs return to power, they re-invite William&Mary's son (again, who doesn't exist iotl, only ittl).
By this point, the fighting is more riots in the streets than civil war, and precedent has been established that a change of party means a change of monarch.
This evolves into an electoral system where 'monarch' is part of the whole electoral slate.
Really, really unlikely. You'd have to have JUST the right people at just the right times, and the civil wars to be, well, civil. But barely possibly, IMO.