I mean, at some point you're just asking me to implement it for real, but I'll give you some hand-wavy answers:
You save your preferences the same way you might save `jquery.js` on your computer.
Chrome IS the compiler, you don't need to compile them first to give them to Chrome.
I'm not sure what you mean by "compiled into Chrome". Since when you do compile a program "into" the compiler? Look, your browser probably JIT compiled a dozen programs just for you to be able to view this page and comment. Did you need to rebuild Chrome?
Yes, Chrome is doing the IO. Importantly, the page is NOT doing the IO. So, there's no XMLHttpRequest, for example.