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.
Last edited by Iceblossom; 08-24-2018 at 11:45 AM.