The variant that we play is that diplomatic die rolls are successful on a 1 or 2 and you can adjust diplomacy positions more then just once in a turn if successful. This is more historically accurate with countries like Austria coming back more than once after surrendering.
Spain starts the game as an ally of France. Thus France cannot capture Spain's capital forcing Spain to surrender. Only Britain can try to change Spain's diplomacy status through political alliance die rolls or capturing its capital. France would rarely use RP to have alliance die rolls but could if it wanted to change a neutral or ally of Britain. It normally is not cost effective for France to do so.