I believe the stepped down rails also aid in reliability as they better shed dirt and crud that could otherwise foul up between the bolt carrier and the rails.
The G2 double hook trigger will work just fine with the NDS3. All that is required is cutting another groove for the second hook.
My NDS3 Romy build is one of my favorites. As mentioned above it was by far the easiest I've built with.
here it is with it's sisters. NDS1P on top, OOW middle, and NDS3 bottom: