At least we learned how to make the correct pattern if not the easiest way. About as back-arswards as you can get.
And a quick way to split that up:
mirror the entire boundary blend
merge it choosing the option that removed the bad slopes
use remove surface and use "leave open" and "keep removed surfaces"
trim (split) the opposite triangle
place the flatten point ref on each slanted line where you "removed" the surface
And unfold each half ...again.
The removed surface becomes the filler for both the formed and unformed feature.
