Microsoft actually reversed course on this. You can make a one-time purchase to access "developer mode" and then run whatever you want. It's been suggested that this is the reason there's been less interest in hacking the Xbox. Ironically it also means you have more computational freedom on the Xbox than on the iPhone/iPad.
Not quite. You (were? I don't know if this is changed) limited in how much of the hardware you could access: it wasn't 100% access. Enough for most homebrew, emulators and so on, but it wasn't carte blanche "replacement for a dev-kit" access.
No it was quite different (again, unless they changed it? They might've, I'm long out of this scene). UWP apps had a lot more restrictions on them, the SDK was different, and you didn't have the full consoles hardware to use, compared to the "behind the NDA" SDK: it'd run slower, basically.
