There is no "SLEEPING" animation. SLEEPING puts "zzzzz" over the head of a character (= effect), but it does not make them lie down.
Regarding your questions:
1. Maybe try it on CharacterStatusAttempt? Although I'm not sure why an animation override would fail once the status has been applied
2. No idea why that would happen. I don't immediately see any script that would do this, and a cursory look at code only reveals the removal of the SLEEPING status if you are hit and the hit does more than 0 damage.
3. You cannot directly play this animation afaik. This animation is automatically used if a character gets the LYING status. However, this LYING status, just like the SITTING status, is not a status you can/should apply directly. Instead, they are tied to using items with a Sit/Lie action tied to them and sockets to which the characters can be attach (like beds and chairs).
