I have to admit... a lot of these rules make my head spin.

On their own, you would use Lethal Hits when trying to take down a tough unit or single model. You would use Sustained Hits when trying to wipe out squads of infantry that are lightly armored or if your weapons have sufficient strength to do damage the target.

If you were allowed to do BOTH, would this then mean that you score an additional hit with Sustained Hits 1 and one automatic wound with Lethal hits? Or will some people argue that they scored 2x automatic wounds?

So the rule does seem to state "If it does, select one of the following abilities" - so I would assume it can't do both. I agree that it's not clear if you can use Dark Pact twice if the model shoots then fights in close combat.