I´m not sure whether I get you right, basically you wish for a diplomatic option "break alliance with faction", am I right?

I agree, that would be very usefull, dito an option to negotiate a peace between two factions.

I don´t get the second part of your post, though. If you´re allied to one of two factions who are at war with each other, you can always send in a diplomat to end your alliance and then offer an alliance to the other faction, always assuming you´re their only ally (otherwise you might get caught up in the delicate web of inter-faction relations).