It is not at ALL impossible to shoot a bag smoothly in midwater, without losing buoyancy, and without dropping the spool. It's more than possible . . . Even I have managed it. There is absolutely no reason to let go of the spool, unless you like to watch it dance; I always worry there's going to be enough current, either where I am or on the surface, to pull it away from me, so I never let it go.
It's my personal opinion that, if you can't manage your buoyancy well enough to shoot a bag and not lose it badly, you shouldn't be doing dives that require shooting a bag.