Wasn't there also the question about the ceiling height with the Pax at 236 cm height? Building a house like that and then having such a low ceiling height that you couldn't even assemble a lousy wardrobe lying down? I would definitely reconsider the ceiling height, it's way too low!
Since during the last move we had to assemble the PAX standing up out of necessity, I can only say: With two people, Ikea experience, and the appropriate height, it is no harder/unpleasant than assembling it lying down.
Yes, I have done that before as well and it worked well. My comment was more because the question came up and the general problem is not solved: the ceiling height in such a house design is a no-go for my taste.