Didn't see this addressed anywhere else (nor in the rulebook).
When you win the Capture Prisoner battle objective and remove an enemy unit from the Attrition objective, does that reduce the amount of units your opponent will kill by 1?
It seems like it should, since the battle objectives are evaluated in order, but the rulebook's language is somewhat unclear on this.
Correct, minis removed from objectives during another objective's resolution will no longer count towards their assigned objective.
What are the 4 extra colored unit stand rings for each of the non-Trog races for?
The Trogs have 16 minis, everyone else has 12, the 4 extras are likely there due to logistics.