As for the impulse engines, as intriguing as that layout is, that's a helluva lot of room to be taking up.
Only by TNG standards. Compare to how much living space the Apollo rockets had compared to their engines (not fuel, just engines). And these puppies have to push the ship to near lightspeed!
Regarding Transport Rooms:
For years I stuck doggedly to the opinion that there was only one Transporter Room on the original Enterprise. After all, Kirk only ever said
The Transporter Room and everyone knew where to go.
However, after scrutinising nearly all the episodes in greater detail, the only sensible way to account for the differrent appearances is that there are
at least two different Transporter Rooms in the series (and perhaps another one in WNMHGB, due to the different corridor).
Because our heroes always knew which way to go and which 'transporter room', I fankwank to say that only 1 main transporter was kept online at any given time while the other 2 or 3 were powered down, undergoing overhaul, or were being calibrated again to come online. I see it as one of those technologies that only has a certain number of 'cyles' before you have to do teardowns to see what's broken. And you'd want to limit the number of cyles each unit had - so you'd rotate - so all would be fairly ready if they had to be put to use at the same time (you wouldn't burn through 1 transporter and then go to the next, you'd want them to evenly wear like car tires).
So every duty shift knows what transporter would be designated as 'active'.
That's pretty similar to my own solution on the matter. After all, there's a tremendous amount of energy being cycled through those Transporter "circuits", why
wouldn't they need regular repairs and replacements?
However, I think that the time it would take for an overhaul is pretty much the same as the lifspan of an active Transporter Room; this restricts ship operations to having one on the go at any given time, which explains why there are so many story situations which depend on there only being
one Transporter.