First, a caveat: This is intended as a warm-weather quilt. I have other 20deg quilts to use when it gets cold, so this will only be used on warm campouts (probably down to upper 40s) or will be stacked with other quilts if used in the cold. As a result, I'm willing to sacrifice a bit of warmth on this one if it increases functionality.

I have a blueridge quilt (similar to the infamous Costco ones, but $50 per quilt on Amazon instead of $50 for a pair at Costco) that I want to turn into a quilt that can be used either as a top quilt or an underquilt. I have removed all the horizontal seams to allow it to puff up a good bit. I also cut off 10" from the width of the quilt and sewed it on at one end to give me a 50" x 78" quilt similar to what was presented here: https://imgur.com/a/kOXYm
I also have the 10" x 15" piece to attach on the bottom of the footbox. Coming back to this in a minute.

I will attach 1.5" grosgrain on all 4 sides to make cinches at the ends and suspension along the sides. Now to my questions:
1) I'm concerned that having an extra rectangle of quilt hanging off the end all the time will negatively impact the ability of this to act as a good underquilt, especially since I'm not sewing differential baffles on this thing (really not possible to do after I've already shrunk the width to 50"). So I think I want to be able to remove and reattach the footbox end. I was thinking about putting KAM snaps or something similar on the extra footbox piece so I can snap it on when it's needed. The cinch cord at the end of the quilt can help close it up a bit around the footbox piece as well to reduce gaps.
Am I missing anything here? Is there something better to use than KAM snaps? Will the KAM snaps go through two layers of a quilt if I move the down away from where they are to be attached?
2) If I'm using KAM snaps to attach the extra footbox piece anyway, is that a reasonable approach along the sides at the foot end of the quilt to form the overall footbox? Would it be better to but the snaps in the grosgrain loops?