The Warp Field structure is perfectly sound. I'm trying to follow Roddenberry's rules. Because the ship's nacelles can't see each other at all, there's two coils per nacelle (Essentially 4 nacelles).
Atmospheric flight for a ship of this size would require an immense amount of energy, and if the shields were to give way, the ship would just fall to pieces.
As for the attack profile, I'm thinking with the advanced sensor array and the fact it's going to be in a fleet most of the time, I won't be worrying about this.
The ship will be somewhat curved, but not as rounded as the Excalibur is. I haven't done any work on her yet because I'm still trying to get it down right on paper.