I am not sure if you are already aware of this, but maybe linking the three layers together will do exactly what you are after? When you change anything timingwise in one of the linked layers, like adding a drawing, the same will happen in the other linked layers. Just hit the link icon at the top of the layer column for each layer you want to have linked.
When you specifically add a blank, then AP will add a clone in each of the linked layers. Is that the behaviour you where looking for or did you need it to add blanks and not clones?
About your second point: Did you know about the “key” function? You kan mark any drawing as “key” just by hitting K. You can use that for a few different things, like only flipping between key frames, or only backlighting keys. Or where you thinking of something else/more?