regarding excluding the setsid() case: I was being conservative as I couldn't find a reference of what was and wasn't allowed after vfork.
I found one thing suggesting that on macOS setsid() was not safe after vfork(). But that appeared to be a Darwin-ism. I expect that is not true on Linux as it should just be a syscall updating a couple of fields in the process info. Confirming, in glibc is appears to be a shim for the setsid syscall (based on not finding any code implementing anything special for it) and in uclibc (*much* easier to read) it is clearly just a setsid syscall shim.
I'll make a PR to undo the setsid restriction given we're Linux only.