Can Dunhere use his special ability to attack the Goblin Sniper in the staging area even if there is still another enemy in the staging area? Not sure whos ability trumps whos....

Yes, because Dunhere is not optionally engaging the Sniper.

Yeah "optionally engaging" is only the act of taking 1 enemy during the engage step from the staging area to being engaged with you voluntarily before auto-engage begins. This means the Goblin Sniper cannot be pulled unless it is the only enemy in the staging area.

Attacking or dealing damage through other methods such as attacking with Dunhere or Haldir abilities are completely different.

During the Journey Down the Anduin scenario, Dunhere is a must for me, with only core deck available. His ability is so **** strong, when you provide him with attack boosts, he can kill a lot of enemies and help you win much more easily.