Why have an alliance in the first place? Cooperation.
If one is at war, all should be at war. If one tribe is causing trouble for an alliance well then the alliance should deal with it.

I see no reason to invent a bunch of complex rules to govern who is fighting who in regards to alliances.

The fact that we have to travel real distances means that at best we may get blocks of power, but the same thing will happen without formal alliance. So they just make administering the group that much easier.