Apple in Violation of Their Own License?

In any case, Apple doesn't allow outside contributors, which means they're fine.

That would depend. If the contributor retained their copyright and didn’t assign it, then you couldn’t commerialize closed versions of it without their permission.

License and copyright are different things. You hold the copyright to your own work and offer it to others to use under the license of your choosing. If they contribute, then their contributions come back to you under license as well (typically the same one). This is why relicensing is such a pain in the ass— you have to get basically everyone who's ever contributed to agree to it.

The escape hatch is if contributors are asked to assign their copyright, which then centralizes decision making with the entity holding the copyright. Which works if you trust that entity (eg, the FSF), but it most definitely has the possibility to go wrong if the copyright holder makes license changes down the road which you disagree with.

For your first question: yes. Look at Qt. They own the copyright and can make a custom build for a company and not have to release the code for it. However, if someone wants to use it for free, they have to abide by the Qt license.

If all people with a copyright interest in a project want to relicense or dual-license it, they have full right to do so. If you start a project, and require copyright assignment from all contributors, then yes you can. If you don't get such a copyright assignment, then you have to get permission from all contributors.

If you relicense an existing project, it doesn't invalidate the prior license on the prior version, so any versions you release under GPL can thus be used within GPL rules, unless you have a contract stating otherwise with a specific entity (and if they wish to dissolve that license and use the work under GPL rules, they have that right too)