Fun to get new insight into such old things...
Agreed that McCoy would be the one to go for the human touch, finding about people's names and feelings. And Spock should know just about everything about just about everything; he's as good at esoteria as Data in TOS already. Also agreed that the crew mingling would not necessarily happen all that easily - how many of them would have been aboard the ship for any length of time before the launch? 10%? 15%? They'd have little to do when construction crews still worked on the vessel.
As for ready rooms and beds, Picard's E-D room had a bed - it doubled as his sofa. However, we never see it pulled out to the full sleeping position, although some episodes show it (accidentally) pulled part of the way. Archer of course had a room right next to the bridge, complete with a bunk, so if Kirk did not, it's an in-universe hiccup in overall continuity.
As regards cabin shuffling, we saw that happen in, say, "Elaan of Troyius". We could assume it also happened in "Journey to Babel" where Kirk uniquely lives down on Deck 5, rather than on Deck 3 suggested in the movies or Deck 12 mentioned in the earliest episodes. If Kirk's normal cabin is on Deck 3 (as Spock's is in ST2), then there's little need for a separate Ready Room. Pike's cabin might also be on Deck 3, judging by certain set details...
Where is the dressing-down room of ST:TMP located? Supposedly, the corridors in the saucer were color-coded to indicate decks (or at least there is some variation to the lighting of the corridor set in the movie, and Shane Johnson in his semi-fictional guide claims the color-coding was a feature).
FWIW, the room we see could well be Decker's room without being the Captain's Cabin; the scheduled completion of the refit was so far away that Decker might have chosen to live somewhere far away from the noise, or close to the action, instead of at the formally appointed location.
Timo Saloniemi