Battles are realtime and you decide which unit of some choices will be sent to each "slot" on the field (units dont move around).
If a unit dies you choose which one to replace it in that slot (both melee and ranged slots exists).
What exactly do you mean by this? I'm a little confused by what you envision for this project. Do you just mean to have a couple of set positions for your units and then they battle it out from there once they are ready? I.E...
(Left Flank) [ Left Wing ] [ Center ] [ Right Wing ] (Right Flank)
[LC Back] [RC Back]
Perhaps I'm very mistaken, but you said that they wouldn't directly control the units, or did you mean individual men and that they control the groups as a whole.
During battle, only the player involved in it are "locked" into controlling the battle, other players go on as normal.
If another battle is triggered but one of the players involved are already locked in another battle, the new battle will be resolved once player is available (the two armies will be locked while waiting).
Personally, I think it would be interesting to implement in a full real-time way as the previous poster suggested. Why lock battles? Granted, it makes it easier to manage from a player perspective, but in RTS games, if a player is attacked from three sides, they learn to cope. The other players are dealing with ordering their troops as well. It could be a strategy to overwhelm the other player, making them choose which battles to focus on and order about manually. Others, they'll have to rely on defensive positioning so they can either deal with the bigger threats on other fields, or simply let those armies be crushed while they hopefully win elsewhere.
I don't know what kind of game you plan on creating, but if it's a many player game (4+), having things like fog of war and observer units to see the battle fields may give players an edge. If you spot them already involved with another player and in combat, now might be a good time to sneak attack and strike an army / town while they don't have the resources to devote to all of their battles at once, increasing the attacker's odds of victory.
With this, you might be able to draw out the length of battles a little bit, and slow down army movement on the overview map.