You can scratch point about "diplomatic battle" , there simply is no such thing in this game. If you become protectorate of someone, you can bet, that it will last for not more than 3 turns until they betray you, and be it only by blocking one of your harbours. The only good thing about becoming a protectorate, when offered to you, is, that you can earn a lot of money with it. Start with 20 or 30'000 Mnai in your counter-offer