I have a question about his special ability that lets him drink potions during the monster phase. Does this also cost a companion? i.e. are you just swapping one companion for another or are you resurrecting a companion?
The Half Goblins Specialty doesn't remove/change the costs of opening chests and quaffing potions. Thieves can still open multiple chests, you can still use companion treasure tokens, etc.
Also, remember that when you quaff potions, you quaff ALL potions. So, if there were 3 potions, you could be changing a single Fighter into a couple Champions and a Scroll for example.
What he really lets you do is quaff potions to swap units into what you need during THIS monster phase. Or pull treasure in a desperate attempt to get what you need. Or to loot the room before using a Ring to escape the dungeon, in spite of the Monsters or a surprise Dragon attack.