One day campaigns?

I’ve been doing some deep thinking about multi-player campaigns,
and here are some of the issues that will need to be addressed.

Let’s for the sake of argument call each strategic map move ‘a year’.
What happens when the Red player invades the Blue player, but the Green player invades the Red player?
Here are some of the possible outcomes:-

* the Red player’s ‘main army’ invades the Blue lands and fights the Blue ‘main army’, while a Red poor
   quality ‘reserve army’ defends the Red land from the Green ‘main army’ attack. And this Red poor
   quality ‘reserve army’s’ composition is decided by the Green player, as if if had been routed previously,
   thus representing hastily raised half trained militia.
(The problem with this approach is the the Red player must fight two battles in the time it takes other
 players to fight one battle, slowing down the whole event as other players must await the outcomes)

* But what happens if both the Blue and Green players invade the Red player simultaneously?
   If they are not formally allied, then Blue and Green will fight each other in the Red land?
   If they are allied, then perhaps both Blue and Green chose say 7 out of their 12 elements,
   while their other unengaged elements are assumed to be protecting their lines of supply.
   This way the Red player will only need to fight one battle in that campaign year, and will
   be slightly outnumbered, with 12 elements facing 14.
(Since the allied army will have two commands, and two PIP dice rolls, it might be better
 if each allied PIP roll suffers a -1 to lessen this great advantage, representing disunity, 
 and each allied command is individually routed when it loses 2 element equivalents)

Just a few things to consider…