1) There are exceptions for entertainment and sports:
Many jurisdictions recognize an exception to this rule, where the contract is either to provide necessities for the minor (e.g., food, clothing and shelter) or where “the contract is for a purpose benefiting the minor’s employment or business” (Valencia v. White, 134 Ariz. 139, 141, 654 P.2d 287, 289 [App. 1982]). Under this rule, sports contracts might be enforceable as long as the contract is “for the benefit of the minor.” That means if the court thinks the employer/team/sponsor has taken advantage of the minor, then the contract will not be enforceable. These are judgment calls, and each case is decided on its unique facts.
Also, contracts signed by a minor can be confirmed at age either explicitly or implicitly. You can Google that yourself.
2) They are paid a stipend and also get college scholarship. I posted the OHL contract already. Plus you give up certain rights when you sign a contract. I had to sign one for a fellowship. I also signed 2 contracts because of signing bonuses, along with various NDAs, and also non-compete agreements.
3) You're not saying anything new, the transfer agreement depends on which league the player was drafted from.
Every league has a transfer agreement with other leagues, so even if a player voids the contract, where is he going to play? The only league without a current transfer agreement is the KHL. Good luck with that. The NHL will honor the transfer agreement if Wright quits the OHL, so he'll be flipping burgers at McDonalds, not playing in the AHL.
The reason why the player agreements are on the web is because there have been multiple lawsuits against the CHL. So far they've held their ground.
Edit: Also, at this point, Shane has a contract with the Kraken, so he has to play where he is assigned. The Kraken have no choice but to abide by the transfer agreement if the OHL does not grant an exemption. So it's not even his choice, hence the contract signed as a 15 year old doesn't matter that much.