1- nope, skills change only once a year, around last week of December
2- read the file "Data\HowToMod.html", and this topic :
3- yes & no : yes, all skills are used by the score simulation; no, some skills aren't used in the real-time game engine
4- neither of that; they are relative to each other, and then normalized according to the current RankPerYear to give the strength needed to reach the RankPerYear ranking.
5- No problem, you can put 5 players n°1 the same year, then they'll be all more or less of the same strength, and u won't be able to predict which one will be n°1 at end of year ; also, the game will be harder for the user, coz he'll have 5 very good players to beat to win tournaments.