On an average what percentage of games are won by a player 100 points stronger than his opponent?
64% according to: Calculator for
Theoretical Probability of Winning Based on ELO Ratings:
http://www.bobnewell.net/nucleus/bnewell.php?itemid=279
How much does his rating change if he has been playing for a long time in the FIDE system?
See FIDE.com for an explanation of how they calculate ratings. The experience factor or K factor is something that recently changed. https://en.wikipedia.org/wiki/Elo_rating_system
At what rating difference does the number of points gained from the higher rating player become zero?
Depends on what system you are using. ELO, Glicko, etc.
See also:
http://en.chessbase.com/post/the-elo-rating-system-correcting-the-expectancy-tables
On an average what percentage of games are won by a player 100 points stronger than his opponent?
How much does his rating change if he has been playing for a long time in the FIDE system?
At what rating difference does the number of points gained from the higher rating player become zero?