Well, let's not forget that "Mr. Scott's Guide" assumed the A was a newbuild.
I should also mention my philosophical approach for warp technology and the way I'm thinking through this warp stuff.
My thinking on Excelsior
reflects an evolving viewpoint (on my part) that assumes that for TOS, and possibly even for the refit, that most matter/antimatter reactions took place in the nacelles, with interconnectivity between them and a smaller "control" reactor in the hull. For TOS, we have enough references to matter and antimatter in the nacelles which, I think coupled with the stated intent of Mr. Jefferies to keep dangerous things away from the crew, and from TMP the intent of Mr. Probert that the TMP intermix shaft is basically a big power transfer conduit, that we can see the potential for main reactions happening in the nacelles themselves. For TOS, we do also have the idea of a reactor in the hull from episodes such as "Elaan of Troyious," hence my notion of a "control" reactor that is used to regulate and/or jumpstart the nacelle reactors. I think the same pattern was probably true for the refit... just what was at the bottom of the intermix shaft? The main difference in the TOS version and the refit would be greater interconnectivity in the latter, and possible and overall streamlined setup.
Obviously, ENT and the TNG era are a bit different. For ENT, we have an overly complicated system that seems to put a simple boiler-style reactor in the hull, and then somehow accelerate the plasma to the nacelles. I could see, as the design simplified, moving the main reactions to the nacelles. Later, TNG seems to move the 'acceleration' component to the matter and antimatter intjections, and go for a more focused reaction chamber.
How do I reconcile Bussard collectors with this? Simple. I think that they existed on ENT primarily as a way to aid in the acceleration of the plasma in the nacelles. By TOS, these "matter intakes" did more or less the same thing, and the same for the refit - until apparently being dropped from the Excelsior
design. By the time they were re-added on the Enterprise-B
, I think they had taken on the "simple" fuel replenishing role we see in TNG and beyond.
Anyhoo, hope that explains things a bit for where I'm thinking from.
As one final thought regarding the Excelsior
chasm - do you suppose it's possible that the ILM modelmakers took notice of the blue glowing windows on the refit that we traditionally accept as a botanical garden, and designed the chasm as an expanded feature of this, which they took to be some sort of engineering component? Just a random thought.