Hey guys ive loved reading your views on this subject. so much i had to join to comment with you all.
So im not exactly very knowledgeable on many trek teck aspects however i would say you are all mainly correct.
The nacelles will move to help create a more streamlined slip space bubble as well as use compression field theory to lower energy requirements allowing the 9.75 warp factor. could bewhy the Promethious has 4 nacelles as well as to allow each parts individual warp capability.
However i would say they it wont have anything to do with in atmosphere as they only use the thrusters in high output mode for thrust. (every time they go in atmosphere they have to configure the thrusters)