1) That is the entrance to the famous brothel. Go to the duchal inn adn in the one of the backrooms, there are 2 men talking about it.
2)There are only 2. You have to chose. But there are other solutions
- Kill one and take that bottle and give it to the other one
- Steal one back
- let one die
3) Did you enter the cave a bit south of the inn? The one with the orc, he drops a golden key.
4) Yes, but that one is only available when you're doing the dwarven counsil member.
