...Then again, the refitted ship seems to be slower than the non-refitted one! Or at least we see the refit fly at warp seven at best, even in life-or-extinction situations, while the non-refitted design did warp ten on occasion without seriously redlining and could pull warp 14+ for very brief periods of time.
This doesn't seem to be an advantage worth the massive inconvenience of having to clothe all the engineers in protective suits all the time. Which is why I'd like to think that the presence of personnel in the area where those suits are needed is not common at all, and only happens in the special circumstances of ST:TMP and ST2.
I'd also like to think that the floor cylinders were part of the TOS design from the very start. It just so happened that they were in a room we didn't see until S2! Main Engineering must consist of multiple rooms in any case, or else Evil Kirk or Ben Finney couldn't plausibly hide in there. And plenty of rooms can be seen in TAS... But since this ST2 side chamber did appear after not being there in ST:TMP, I guess refits are also possible.
Who knows, perhaps the room in ST2 is a patch for a piece of automation that failed? Rather than repair the original machinery, which may be badly irradiated and would require ripping apart the entire ship for months, the engineers just opened a new hole in the system and then plugged it with a multi-layered radlock featuring that pedestal as the first line of defense and those transparent walls as the second line. Poor old NCC-1701 might not have warranted better treatment at that time and age...