But for example, could you ship Gecko with Apple's JS engine in a way that will satisfy Apple's requirements? Of course the answer is no, but it's worth taking a second to think about why that is.

I guess what's bothering me is that there is an unstated assumption underlying this entire discussion which people never bring up: for performance reasons, the rendering engine has to be tied closely to the JS implementation. Specifically, calling over some IPC interface at every entrypoint into the JS interpreter would be a performance nightmare. This is the reason why these can't be disentangled. The connection to the interpreter is really an indirect one.