m90 + 2.5 apex quilt

Folding to save sewing one seam wont do anything for you.

well it would leave a seamless edge at the head end and save some sewing..

True, but you'd probably still want to anchor the apex on all four sides. While 3 sides will probably work, it may allow the shell to be pulled away from the insulation and eventually for the insulation to tear away completely (such as when stuffing/unstuffing if extra care isn't taken).

For the minimal weight/time savings it's probably better to sew the 4th side just to increase durability.

OK, that makes sense. also means i can make the inside either taffeta or even M50..

Sew 4 seams. You want a draw cord at the top and either a draw cord or footbox at the bottom. You won't save much by folding it for the 3rd seam but in my opinion you lose a lot

definitely a drawcord at the bottom with velcro too. i'm not sure about the top. i don't usually have things tight around my head/neck. i'd probably add a long sleeve layer rather than have it cinched up.