Bug in sleep.h

Surely that's a timing bug right there...

Correct. From most applications an sei / sleep_mode sequence is a timing related bug (a "race condition").

The code snippets in the Libc documentation show the correct way to put the processor to sleep (sei immediately followed by sleep_cpu)...
http://www.nongnu.org/avr-libc/user-manual/group__avr__sleep.html

I'm not sure what the process is for writing these libraries that come included...

Which libraries?