Try putting a different browser renderer on Firefox OS. You're pretty much locked in to JavaScript — unless you can somehow build WebKit with Emscripten, you're already behind what iOS can do — even with Apple's rules.
It's not about replacing the browser, it's about being able to replace things that you should be able to. There is no good technical reason to not let people replace Safari on iOS.
(I'm guessing "probably not", "no", and "yes".)