Grand Rapids Area Boardgamers
First: Can a surfaced dragon enter tiles with other players in them? Knight, Goblins, Thief?
Yes. Page 9:
While surfaced, if you enter a tile with another player, resolve an attack as if that player attacked you.
Second: If you are not playing with the Ash Dragon variant, can the Goblins enter tiles with a surfaced dragon? Can they enter tiles with the dragon underground?
Page 7 implies this is yes (this is why you'd be "passing through"):
To enter or pass through a tile with the Knight or Dragon, the Tribe’s Strength must be greater than the Knight’s Strength or the Dragon’s Armor.