Lady Diana

Not sure You can Wash velvet. Spray Adhesive won't stand up to washing at all in fact it will disappear. That Would leave your Front and back sections loose.
you could quilt the front and back separately with no batting. Secure the quilts along the edges and make a duvet. Then slip a thin comforter inside. W h en making the duvet, create a flange at one end to button along that side.

sewingpup

I think the tacking/tying idea would work pretty well. I don't know what the top looks like but you hopefully will find some areas where a simple tack or ty would work well. Maybe even little bows or buttons could serve to tack together depending on the design.