Not sure if you're having the same problem, but I had an issue where I had two diners merged into a restaurant, and a third diner next to them I couldn't get to merge - turned out I had upgraded the two merged diners, but not the third. When I upgraded the diner that wouldn't attach to the point where they were all the same level, they suddenly merged into a three-unit room.
Yeah, i've had a similar thing , I think it depends on the level of the rooms. I think they have to be the same level to merge.
Yup, only same level rooms can merge. Upgrade your lower level room and they will merge.
Yeah the rooms have to be the same tier to merge. So you can upgrade the room after placing it and it will automaticly merge to the other room when tiers match.