1. Generally to a league 1 level above you, but sometimes there's a jump to a higher league, as mentioned in other threads. This seems to happen more ofetn at higher levels, so you shouldn't worry yet.
2. The value goes down, the players are older and you get less money for them. I wouldn't recommend getting rid of your players before you have proper replacement and it's really hard to buy someone in the first days of a new season.
3. Correct, 2500 people.
4. One youth you get for free, you have to sign a contract with them for 0 tokens, 0 money. The other is worth tokens and no money. The free one is of lower quality and in my case they are no good so I sell them for extra cash. I only signed one youth player and is he worth 24 tokens I gave for him? Fast learner, good quality, but 24 tokens cost me one season with only one new player. I'll stick with the auctions in the future.