I think the joke is that in all cases but the best-case, you won't be around to observe it, this becomes practical once you can create a universe for every permutation and destroy all but the universe which notifies you that it's the best-case.

In all good text books this is left as an exercise for the reader.

In a really good text book, this is left as an exercise that the reader won't do.

No, checking whether a list is sorted is O(n) ;).

Bogosort is O(n) in the best case and O(inf) in the worst case).

Optimize out the check, so it will be O(1) in some cases. ;)