Not sure if this forum is still up running since Descent SE is up. Nonetheless I ask.

We are playing rtl and approaching the avatar keep. We are wondering if the lieutenants will reinforce the avatarkeep if they are 1 trail away from the keep, the same way a lieutenant would reinforce a normal dungeon?

Yes, you will get the bonus from any LT in the same location or one trail away.

However, when the final battle begins you will have to discard the Overlords deck, including any power cards in play regardless of other effects (ie - LTs)