request for clarification of the rules

[bravlyon]

Hello everyone,
apologies for my English – I'm writing from France, using a translator. I'm having trouble with the movement sequence, particularly the 'move to exploit', which is triggered during the movement phase, whilst the breach in the enemy's defences occurs during the combat phase. What's the point of having two moves during the movement phase (when the unit intended to exploit the breach will surely be positioned just behind the attacking units) and only one move during the combat phase to exploit any breach?
Secondly, if the unit intended for exploitation is already behind (adjacent to) the attacking units, how do you set it to 'move to exploit' without actually moving it? If I click on it, then select the 'move to exploit' order, I'm forced to change hexes to gain that status. If anyone could explain this mechanism to me, I'd be very grateful.
Another question: how many casualties can a unit sustain before it is eliminated? I haven't been able to find this information.