You don't want units to have different movement values precisely because we don't have simultaneous attacks. Bone troll (60+ attack, low initiative, low defence, high hp) is dangerous enemy because most units have only 2 movement. If your units move at speed 3 or more and your initiative is 10+ then you can avoid troll attacks. You won't ever get hit when you have movement/initiative advantage and when you know that you can kill that troll without ever beaing hit you will start running around map and waste MINUTES trying to avoid his hits.

Increased movement may amplify the AI's problems.  In my experience when the AI has fast units they run off and leave their allies.  Essentially you get to kill the enemy in waves using the process you described.