I am not sure I follow you.
The MP moving last wont be the only one phasing - all the MPs in the combined movement block will be phasing. Thus I fail to see why it would be a main problem if some battles are triggered by someone other than the MP moving last in the block.
The phasing player is the one who decides the sequence of battles, likewise he is the one sending files.
You can not have simultanious battles in a pbm game, since all nations need to be in the same sequence of the game.
Whereever several MPs control forces on the same side in a battle one of those MP must be picked to run the battle for that side (should be done by the computer based on leaders present and/or troops present). Once thats done you run all combats the same as you would otherwise. Sure, this means that one of your allies will be deciding loses for you - but thats no worse than loaned corps.
Regarding the combinatorial issue you mention its not clear to me what you mean. Are you talking about the issue where not all major powers are at war with the opposing forces or exactly what is it that you are seeing as a problem?
No, lets say the mentioned nations all fight France (Aus/Pr/Ru/GB).
GB might same time fight a Spanish controlled Naples, and Russia could be fighting a Turkish Georgia.
Since all battles commence after all (Aus/Pr/Ru/GB) made their land move.
Aus/Pr/Ru could have 1 joined army in the north, Austria a Au/Ru army in the middle, A Pr/Gb could be in Holland, alot of solo corps could also be involved in siege battles or fighting solo corps.
Who decides the order, when it can not be done simultanius.
Hope this helps.