1) Your player's salary gets adjusted to their skill level at the end of every season. I assume your PF got trained, and his skills at the end of the season were much better than at the start. So when the salary adjustment came in, his salary jumped. in short, his salary increased because his skills did.
2) "Best" is subjective. It depends on what you want to use that PG/SF for. I'll just cover this generally.
A PG's skillset should cover Jump Shot, Jump Range, Outside Defence, Handling, Driving and Passing.
An SF covers all the skills, with emphasis placed on outside/inside skill depending on the orientation his manager intends to play him. A balanced SF will have equal skills in all departments, though I think Shotblocking can be sacrificed for other areas.
3) Educated guesses.
We could change the topic, but then someone will have to create a new one.