Sorry for the delay in responding to your questions.
CORNER CATENARY CURVES: The corners do have a catenary curve, and yes that does reduce interior headroom. Raising the shelter up a bit helps a lot to increase sidewall headroom. It snowed on the shelter only one time when I was testing it. It was about 1" of wet snow that stuck to the tent; it really flattend the lower sides of the shelter and did not readily come off. It was not very inhabitable under those conditions. I did not use the shelter for camping in dryer snow.
CUBEN FIBER WEIGHTS: I incorrectly stated that HMG uses a heavier weight of Cuben Fiber. Upon further research I find there is some variation in the stated weight from .74 to .78 g/sm, but in fact they are all exactly the same thing. Only ZPacks uses a lighter weight. CT2E.08, about 26g/sqm, .75oz sq.yd, break strength 105 lb/in is the Cuben Fiber used by most manufacturers; CT1E.08, about 18g/sqm, .53oz sq.yd, break strength 68 lb/in is the one used by ZPacks.
SEAM CONSTRUCTION: The four corner seams have two lines of stitching and are taped on both sides with 1.25" wide Cuben tape. See photo below.