Only if they are French, then they cant score.
No. No. Yes.
a bit of both. Because your teams loses a star each season, you will need to bring in new blood, especially when powerlevelling or POWERTRAINING (PT), where you will get rid of slow trainers.
Level 1 through 3 is fairly simple and you will have no problem buying players. You will have a problem SELLING players as nobody usually wants your bad players.
Read more on auctions and when to buy players and make your own mind up because everyone does it differently. I buy 5 star players at the end of the season knowing they go down to 4 star, but those 4 star are usually young players and will gain it back fast. not only that, but you save tokens by not getting into bidding wars like in the start of EVERY season. Buying a 5 star on the last day of the season is better than wasting a crapload of tokens on the first day buying a 4 star. But if you are PT, then you have tokens to waste, so you can easily waste them buying 18yo 5 star players on the first few days.