if thalin is exhausted during quest face you deal a damage to enemies revealed from ecounter deck. got it. what about these goblin spearmen and swordsmen when they're revealed as a shadow card and go to the staging area. how long is thalin considered to be commited to the quest. end of quest phase. or end of round?

Only until the end of the quest phase. After that he is simply "exhausted" as the questing phase is over.