1, 2, 3 - You would have to check Steams API documentation. Twitch don’t handle how the reward is distributed, that’s up to you and however you wish to distribute it and request any connections such as to Steam from the user should that be needed.
Twitch doesn’t know what a Drops reward represents, it could be a beta key, could be an ingame item, could be anything, they just handle who has earned and claimed a redemption and pass that data to you and you need to handle fulfilment of that reward.