Q: At the Warlock's Cave, do you accept a quest...only when you land on it? To get a Talisman after a non-delivery quest is completed, do you still have to return to the Warlock's cave?

"A character who lands on the Warlock’s Cave
may choose to accept a quest, or not, as he wishes."

So yes , unless you draw another card that gives you a
quest you have to visit him.

The space itself says: "When you complete the quest, the
warlock immediately teleports you back here and gives
you a Talisman if available."

Alright, thank you.