it takes several casters

Several first-year casters. Quirrell stuck 50 people to the ceiling. You might say that he has better ways to use his power - but the killing curse is not one of them. His killing curse is little better than anyone else's.

Also, it seems unlikely at best that the Hover Charm can accelerate people downwards

There's got to be a spell for that, and it it likely to work similarly to the hover charm, i.e. instant effect.

Also, it's possible that the Hover Charm could be blocked by a pre-cast shield

If by a specific anti-hover shield, then one needs to always keep up a large number of shields against various spells. If by a generic shield, well, that isn't in my mental model of shields, but I guess it's possible. In which case I agree that the killing curse is superior agains a raised shield, but it is still inferior against an unsuspecting enemy.