Do not mix up the level/time scale of play from CMC to CMBB.
In the early days when only CMBO was out I got to play in a fairly big battle with about 40 players (its been a while) :confused:
All the problems were worked out. I am guessing that is how CMC may have been born.
As for the scale of CMC your units occupy each 1km X 1km unless you are moving so the setup will be based on your status. If you are stationary then do you have scouts out to alert you to pull back or dig in. You may have chose before to dig in where mine fields are laid and barbed wire is strung out (defense) and it takes time.
As for leaving the battle field before the game is over, make sure that the battle field command and control does not cause a route of all units still in the battle. It would be a tactic to leave a defending-delay unit so you can pull out. I think this will be part of CMC that will take a % of units for a short time play (less then 60 min).