A round head hole is harder to sew and will require a tall collar with a drawstring to seal, and the collar will likely be thinner than 2.5 to 3 " loft of the quilt top. A 23-24 inch slit opening (try and measure before you cut, you can always open it up more if need be)can be sealed along it's line with no-snag velcro. A flat collar will tend to fill the pinched cold spot that the slit forms and probably be warmer than a round head hole. The slit opening may not be optimally located halfway down the bag; try mocking it up in plastic of cheap fabric.
2.25 inch sewn baffle height will give you almost 3 inches of loft in the puffy center if you have enough down. At that thickness, a differential cut might be of some use. Also baffles that curve. Anything to allow the down to loft freely. No-see-um netting is knit, so it shouldn't unravel or need ends hemmed. If you are just stitching the liner and shell together you can trim the netting that sticks out. I recommend that. Sewing end baffles is hard and the pinched edge doesn't loose too much warmth.
12 oz of 800 fill down should loft freely to fill 9600 cubic inches. In practice the down may not reach that free loft if your design put's stresses into the shell. I assume you are using 800 fill down from thru-hiker. 12oz in a quilt can easily give you 2 -3 " of loft. How warm the quilt is at those lofts starts to be dominated by volume and draft resistance, how well sealed, your sleeping style, pad insulation, etc.
I hope this helps. Good looking house wolf ;-)