if youve got the ability to cut/route the concave neck profile into the body, then why not do that on the single cut body shape only and keep the flat heel section of the neck the same as in your single cut neck shape? if that makes sense???
so for the double cut version you have a slight flat heel at the neck/body joint and it will give you more wood/thickness which you need in the weak spot.
for the single cut version, you have what is shown in your 2nd drawing, but with only a slight area of heel on the treble side. You could then route the body with the concave neck profile and let it wrap around the neck. You may need some kind of thin compressible material in there so theres no gaps and it feels solid. It would also keep the neck free from scratches if you did want to swap it over to a double cut body.
I hope that made some sense, I can picture it in my head but I doubt Id be able to draw it