I made a variable insulation quilt last year for testing. Instead of what you described, I built a conventional 5osy XP quilt, and I left the top edge unsewn. The idea was to be able to insert a 2.5osy layer as needed.
At the top and bottom of the quilt, I used silk pockets sewn to the edges on the bottom and sewn to the edge and top of the insulation, which allows me to turn the whole thing inside out, insert the 2.5osy layer with the top and bottom in pockets, then turn it right side out again.
The results were interesting, but ultimately left me with a feeling that it's not worth it.
1) It works fine, with virtually no extra weight
2) I cut the extra layer into 3 strips along the length to test if a variable amount of insulation on the top but not the sides was worthwhile, and if there would be any shifting of the 2.5 layer. The layers of XP seem to interlock enough to make it not matter, and even using adjacent strips didn't cause any issues for the target temps (not pushing the edge)
1) Leaving an edge open is a hassle. It works, but a more practical solution would be to have a zipper or to hand stitch a straight stitch that can be pulled free when you want to swap insulation.
2) The pockets were useless. The silk had a tendency to pull out, and probably wasn't necessary since the XP seems to self grip fine.
3) Given the costs involved, I'd rather have dedicated quilts, built normally. Simpler, less to go wrong, and less to manage in the field. I know my tentmate would have preferred I had a simpler quilt and not made so much noise occasionally managing my quilt.
Looking at your idea, you could optimize it a bit by skipping a shell on the insulation, and just use an edge of silk sewn onto the perimeter, then perhaps use two or three strips across where the velcro dots are to help stabilize stresses. Velcro to the inner side of the shell rather than the outer shell. I like to leave my outer shell slightly loose to allow maximum loft, and use the inner shell to transfer any stress (shifting position, tightening down the quilt, etc). I don't think using a shingled approach is going to net you anything but construction hassle. Climashield is better off untouched.
I like to sleep extra warm (a little below sweating, honestly), and will take 5osy up to 70 degrees since my quilts can be opened flat. I prefer 7.5 total for ~20F, but have done fine with 5.0 and a layer of fleece on my body. Ron's descriptions for his Spirit quilts roughly mirror my opinion on warmth.
My variable insulation quilt is built with 5osy, and I can add whatever I want to it without worrying about velcro or other attachments. If you want to switch between 2.5 and 5.0, I would recommend building a normal 2.5 osy quilt with a straight stitch closure or zipper/thin velcro (your preference), then have a cut 2.5 osy layer you can insert for winter. When I wash my quilts, I do it by hand in a bathtub. Packing it in the field is probably a lot more rough on the insulation than washing it that way, so I don't think worrying about a seperable shell is worth it.
Basically, if you avoid my mistake of leaving an edge open, your idea of a variable insulation quilt would work fine, but I think you'd be happier by using a mostly conventionally built 2.5osy quilt, and inserting a plain cut 2.5osy layer for colder temps and having a simple zipper or straight stitch closure.