Lords of Waterdeep

No — you must place an Agent on an unoccupied action space; you cannot place on a space that contains another Agent. Exceptions include Buildings with multiple action spaces (e.g., Cliffwatch Inn, Waterdeep Harbor) and certain Plot Quest effects that explicitly allow assigning to an occupied space (see pages 5, 6, and clarifications on page 12).

Yes — assigning an Agent to Waterdeep Harbor requires you to play one Intrigue card from your hand, and if you have no Intrigue cards you cannot assign there. Also note Harbor Agents are later reassigned after all Agents are placed (pages 5–6).

No — you may complete at most one Quest on your turn, regardless of whether you reassigned an Agent from Waterdeep Harbor. The turn allowance is one Quest per turn (see page 6 and the rules summary on page 13).

The action space remains occupied until that Agent is removed; purchasing the Building does not automatically remove the Agent. Owner benefits trigger when another player assigns an Agent to a Building (not simply because you purchased it), and special cases such as Sample Wares or the Ambassador are clarified in the rules (see pages 5 and 12).

A Mandatory Quest from an Intrigue card is placed in front of the chosen opponent, and that player cannot complete any non‑Mandatory Quests until they complete the Mandatory Quest. When the Mandatory Quest is completed it is placed in the Intrigue discard pile (page 6).

Yes — in final scoring each Adventurer in your Tavern is worth 1 VP, and every 2 Gold in your Tavern (rounding down) is worth 1 VP; additionally score your Lord card as specified. See the final scoring rules and table on pages 6 and 13.