The WPA rules include specific rules for wheelchair players. I don't know of any junior player who requires stools or such. Leagues are free to add their own special rules for people with disabilities.
As for four balls to a cushion ... if a player can't routinely make four balls hit cushions on an eight ball break, they need very badly to practice their break and they probably don't belong in a league yet. The alternative seems to be to allow a safety break at eight ball.
It is hardly surprising that you don't know any players that the rules so discriminate against, that it would be impossible for them to play.
As Dr Dave reminds us here, the rules are the rules, not just for leagues.
Thank you kindly.