Ad 2. You don’t need a vulnerability in OS sandboxing. You need a way to start communicating with the browser process, since the way renderer processes do I/O i

Ad 2. You don’t need a vulnerability in OS sandboxing. You need a way to start communicating with the browser process, since the way renderer processes do I/O is they delegate it to the browser process by communicating over a socket. With a sufficiently advanced arbitrary code execution that’s possible without trying to find bugs in the kernel.