At this point nobody knows the exact answer to your question but here are my opinions.
1. stamina is important (at least above 50) the others not so much.
2. by far skill ratings
3. forward (shooting, cor offense, fk offense, heading, recieving, striking) mid (everything) defense (marking, tackling, heading, cor defense, cor fK)
4. just the beginning of the season as far as I know
5. They start to go down around 28 or 29 but playing time matters.
6. not very fast unless they are 21 or lower.