I use 2nds 1.1 ounce silnylon
It's possibile there is slightly more misting with 2nds, but regardless you get some condensation that can be knocked off the inside of the tent that produces more water on you than the misting so it doesn't really matter, in my opinion. Regardless, you should probably have a water resistant sleeping bag, but your body heat will evaporate a little water regardless.
I use catenary ridgelines. See the article in MYOG about making silnylon tent, there are a couple possibly useful ideas there including how to do catenary ridgeline
Flat felled seam is fine.
I always aligh the warp and weft, but I'm not sure that it makes any difference. They typically spec different warp and weft strengths, so maybe they stretch differently which might make a difference. Silnylon is fairly stretchy so it probably doesn't matter. If you're using 2nds, it's fairly cheap, so maybe you shouldn't cut that corner.
Maybe it's more important to align a diagonal cut on one piece to a diagonal cut on the adjacent piece, but even that isn't probably critical because silnylon is fairly stretchy.
We want to see pictures of your finished product : )