1. Stakes mean share in total pool. Suppose ICO decided to give out 100k coins and 10 participants joined and you are getting 5 stakes per tweet. Now when the ICO end, you will get - (You Stakes / Total Stakes as earned by all members) * 100k
Read here for complete info about stakes-
https://bitcointalk.org/index.php?topic=1715881.02. Most probably you need to make wallet in order to receive tokens. Well you can also receive coins on exchange wallet if coin get listed.
3. Tokens are converted to BTC via exchanges. It is possible only when coin get listed on exchange. To know whether particular coin listed at exchange, either read ANN of coin daily or track coin on coinmarketcap.com