Zaharist, I like general idea, but it's nearly impossible to make this system unexploitable. You have to deal with a lot of different situation and possibilities, the most obvious of which is leaving/joining factions.
Also, one of your ideas sounds really awful:
they can't help with roster (before battle sides should choose factions they would like to hire for their side. the only condition is that overall number of players in chosen factions should not be greater then xxx.)
This game is mostly about battles and forbidding people to fight in some battles according to diplomacy system would most likely ruin it.
Nocti, I agree with you, but some people will not fight their friends if they have choice, and surely they will not make alliances with those they dislike. Basically, IMO, personal dislikes, insults and all other forum shit is one of the most important reasons why people keep their alliances from previous strats. And the only way to change this( I don't consider removing diplomacy forum section as option:) ) is to make predefined alliances at the beginning of each round by random drawing.