... does move speed also affect how far a character can move on a single action point while in combat?
Yes.
If one character has high sneak but the other does not, won't you get spotted by the monsters anyway, so you still won't be able to sneak up on them?
You can still sneak pretty far with nominal 0-1 sneaking. But afaik, yes to your question.
The only thing I've been able to do was to break the chain connecting the two chars, have one char sneak up while the other stands far away, but then the battle starts with the two of them really far from each other.
Am I missing something?
If that's the case, and it's unintentional, you're not strategizing properly.
Everyone else has contributed a lot, but here's my 2 cents:
-If you put at least 5 points into Sneaking, the act of Sneaking in battle only costs 1 AP; combine it with Guerilla, and you have a 2x dmg multiplier for only 1AP per attack/action.
Sneaking doesn't actually have to be used exclusively for sneaking =p