What is heightmap used for ?
With your packing scheme above, you are depriving yourself from composite texture and proper mip map generation for your normal maps.
So, unless you are handling it on your art pipeline, your packing scheme is inferior to all other possible alternatives in each and every respect, unfortunately.
Depends. AO maps are primary candidate to get munched out for landscape.
It is not uncommon to bake a little bit of sharp cavity in albedo and call it a day instead.