One quick question: If an opposing force is wiped out in the first round of combat, do Human Knights or Undead Necromancers still have the opportunity to draw tactic cards or summon Reanimated?

Also:

One of the tactic cards forces a neutral force to retreat. Who decides where to retreat - the player that played this card, or the one on his left (as in combat)?

1. Yes, they would sill draw cards in order to summon reanimates or draw tactics.
2. The person who is retreating should decide where to retreat.

1. See above answer

2. It depends on whether they are actual neutral neutrals, of allied neutral units. When they are neutral, I think the player left of the active player gets to place the neutrals. If they are allied, however, the player whose units they were gets to choose (and he has to retreat to friendly areas first, empty areas second).

Just to confirm with #2, Zabulus is correct on who gets to choose. Either the owner (if allied neutrals or faction units) or the person to the active players left (if unallied neutrals).

For #1, strictly speaking you are supposed to draw cards for EVERY unit, even if everything else is dead. As a practical matter, some people just skip doing so except for Knights/Reanimates, though, if the cards don't matter.