Yes, you must play all your cards each turn, including your Stumbles. The only way to avoid playing a card is with cards that allow you to discard cards, such as Sleight of Hand or Apothecary.
Every time you enter the Monkey Shrine, you can pick up one Monkey Idol, so if you enter it three times before any other players do, you can grab all three.
Yes, you can buy as many Market items as you want, including multiple Crowns if you wish or multiple Backpacks if you wish. Each Backpack allows you to pick up an extra artifact, so if you had the good fortune to have enough gold to buy both Backpacks from the market and had enough boots to run around collecting artifacts, you could have three artifacts.
Yes I know about that all cards which u can use to discard another cards, but there is nothing on start If we have only 2 keys and 2 backups in the box, so it could be easy to block other players, especially with keys...