You make some excellent points. I can only conclude that the Hover Charm (and your proposed opposite) must have some built-in limitations, otherwise no-one would bother using anything else against unshielded targets.

I do suspect that the spells we think of as instant are actually very fast invisible missiles - this would account both for the fact that one aims them with a wand, and for the fact that the likes of Quirrell can sneeze away spells which have no visible missiles.