I'm not sure who will be the nominal 'main character', it seems to depend on the reader's perspective, as well as each of us writing. For instance, each of our posts will be from our character's point of view, which means the vermin characters would consider themselves the protagonists of their own life story.
As for who dies, I would suggest any deaths occur very late in the game, for those of us only writing one character. Personally, I think it would be more interesting to have characters survive the ordeal, rather than the losing side facing total obliteration. That way it's possible to develop characters further and show their emotional and mental reactions to victory or defeat.
This brings me to which side 'wins' and which side 'loses'. As far as I can see, the objectives are thus;
Woodlanders/Long Patrol: To retake Salamandastron and rout vermin forces from Mossflower.
Horde: Hold Salamandastron until Woodlander forces retreat.
The Woodlanders certainly have the harder task, and a prolonged siege can be very costly in lives and can be emotionally, mentally and physically draining.
Perhaps it would make the most narrative sense for the Woodlanders to eventually succeed, but at great cost. Perhaps the experience would bind the woodlander characters together very closely, but at the same time make them feel disconnected from ordinary life afterwards. I do not know what would become of Ulath, Viper or Arra, it seems we could die, or flee, and never be found again or be captured.
Personally, if the woodlanders were to win I would want Arra to be captured, as it would allow me to role-play with the woodlander characters, which may lead to some interesting scenes.
I am interested to know the thoughts of my vermin comrades as well as everyone else on this.
Some further thoughts I've had are; how will our characters act around each other and how might that develop?
For instance, on the vermin side, will Viper and Arra get along? Will Ulath start to like either of us? Or will we be rivals, or suspicious of each other? The same questions can be applied to the different Woodlander leaders, all who have beasts under their command that they need to organise and keep alive.
Will different command styles conflict? Will the Long Patrol characters assume seniority in the ranks by being a more organised military unit? Will they look down on Redwallers for being 'soft'? And what about the rank and file woodlanders? Do they all want to go to war? Are some just there because of pressure or fear of being accused of cowardice? Are some just there for hereditary reasons, (eg; Guosim shrews and otter crews tend to also be extended families. The Long Patrol is presumably also comprised of large hare families.) and are only fighting because they fear for their loved ones?
Similarly, is there dissention in the vermin ranks? Do the majority of vermin want to sit on Salamandastron and enjoy the spoils, or do they want to take a more aggressive stance? Did some of them join up just for food and safety in numbers? Were some of them forced? Did some of them join for the thrill of conquest, or the desire to wreak havoc on woodlanders?
Will there be deserters on both sides? If so, will they be caught? How would the horde treat their deserters compared to the woodlanders? Would both exact a death penalty?
Just some questions to think about. I personally plan to have a lot of the inner workings of the horde described in my posts, as Arra is something of a spy, and likes to hear gossip, and what the general mood and morale of the horde is.