This looks good enough to me. I'm not sure if it's true that start is used in majority cases. PATH search is needed for builtin applications, but for regular ones I'd expect users to pass Unix paths most of the time. Still, it's needed for 32-bit/64-bit mismatch (although not in wow64 configurations).
Still, I agree that if we can have mitigations working for more cases, it would be nice. I just don't see a good way of doing that, but I'd be more than happy if we can find a clean way.