Pointers and references have the same layout. Mutability of the pointer or reference does not change the layout.
Does this mean that I can cast *const &mut [T] to *const *const [T], right? And the length of the slice is guaranteed to be at the same offset for both of them (say 8)?
The offset of the length is guaranteed to be the same for &mut [T] and *const [T]. In the case of &mut [T] and &mut [U], say, that's this issue.
Prefer functions like from_raw_parts when possible, so you don't have to care.
This paragraph doesn't say anything about Sized, so yes. Furthermore, the next paragraph does restrict to Sized, so this one wasn't meant to.