Iceblossom

So if I'm understanding, you are only going to use the batting on the "bed" of the quilt, and leave the sides without any?

That sort of thing can be done/used to be done more often than now. You'd basically just attach a skirt to the completed top. I'd want to gather it (at least 2 times the length) and not depend on just falling gently, but your way will also work.

Typically for a king sized quilt I go for roughly 3 yards square, so 108x108". I use batting the full width, sure it may be a bit stiff at first but it softens. Also I wouldn't use any thicker than warm and natural, or a medium/thin poly batt if drape is what you want.

Edit:
Also, in your construction method using one back you should stabilize/quilt the sides at least lightly. Just channels going down every couple of inches would work, otherwise it will probably be a small nightmare after washing.