Only one key per turn, in step 1. You use it either to move the messenger to the other row (castle or village) or to discard and refresh the three cards in the messenger's current row, not both.
No. In the base game a key refreshes only the row the messenger is currently in. To refresh the other row you would first have to spend a key to move the messenger, and you only get one key per turn.
Only up to a point. A face-down card still fills one of your nine tableau spaces, and there is no pass or discard action, so you can take at most nine face-down cards and the game ends after nine turns each.
Yes. Discounts are permanent and apply to all your later purchases, but a discount card does not discount its own purchase. Discounts cannot take a cost below 0.
When played, you put gold from the general supply onto two purse cards in your tableau until they are full. The gold does not come from your own stash.
Yes. The Italian text differs from the English, French and Dutch cards, which all say to fill two purse cards. Players report newer Italian copies were corrected.
No. The Banker has no purse, so you cannot put gold on it. It scores 1 point per gold on the other cards in your tableau, and leftover gold in your supply does not count.
Yes. The Woodcutter gives 1 gold for each occupied tableau space, and a face-down card occupies a space. The Beggar's per-full-space coin works the same way.
Only two ways: a card effect such as the Steward places gold from the general supply, and at game end you place leftover gold from your own supply onto purse cards, up to each purse's capacity.
No. A face-down card has no printed information: it is not castle or village, has no shields, and scores nothing, but it still fills a space. The Carpenter is the exception: it scores when you have a face-down card.
Yes. It scores for every set of three identical shields anywhere in your tableau, across all shield types: each set must match internally, but different sets can be different types.
A key on a lock marks an unused ability. Remove it to trigger the ability, once per turn at most. A key still on the card at game end goes to your key pool and scores.
Those of one neighbor, a player directly to your left or right, not an adjacent card in your own tableau. You gain coins based on the castle cards in that player's tableau.