Regarding double occupancy of a Hi Passage Stateroom, I always figured that a couple would actually get two adjoining rooms. One would be used as the bedroom, one for the sitting room.
Unfortunately, if you look at modern cruise ships (designed for 1 week voyages similar to starships) the size of the stateroom is really very small. Also, there are several more classes of passenger than Traveller proposes. I'm sure this was for simplicity, but perhaps a 4 class system would have worked better. Hi Psg, Mid Psg, Low Psg (Steerage) and Cold Sleep.
The low passage would become 4 passengers per stateroom and cost 2000cr.
Neither here nor there though.
Assuming that the sickbay comes out of the Medic(s) space is a good assumption. I also assumed that a piece of each passenger's stateroom made up the "common" areas, like galleys, lounges, casinos, bingo parlors, and corridors.
I also assumed that the engineering tonnage also includes provisions for minimal repair shops.
Additional space could be directly allocated for something if necessary.