Follow the Map!
These organisms are cleaning out an eye socket infection
While Dave is correct that we’ll need to know which rule set you are using, I think the answer to your first question is generally the same for all the rule sets. It’s PGD (or routed) units that lose an increment for withdrawing from or retreating through an enemy Zone of Influence. Retreating before combat does not treat the retreating unit as routed, therefore it does not lose increments for retreating from a Zone of Influence.
In reality, the skirmishers would be withdrawing simultaneously with the cavalry’s charge. The game mechanics create a sort of artificial start-and-stop, as it’s not really possible to create the dynamic flow of the situation.
Sorry. I'm using M-L rules.