Not sure if anyone's really interested in the details of our search (see blog below), but I am finding it all very exciting and can't resist posting at least some observations, as the initial confusion begins to lead to at least a partial enlightenment.
Looking at the layers of decorative stencilling on the 1st floor, we get information about the age of the walls, and the approximate date of the major reconstruction(s?) that caused such an upheaval to the layout of the rooms and the ceilings. Here you can see an early 20C stencil (floral).
Under it is this gentler green design, then a couple of (pinkish) layers that won't separate but then you get two blue patterns, one on top of the other (the darker blue design is the older). The main partition wall's layers end here. It's a lovely, generous stencil in deep blue, with grey band around the floor level and a grey-blue strip below the ceiling. Our expert dates it to mid-19th C, which confirms the reconstruction having taken place at the time when ceilings had to be hidden or gotten rid of by decree. (Quite why the then builders had to chop the ceiling off at the partition is another question though - but on that later).
The outside wall's decorative layers continue deeper - the reds probably baroque, the black probably as old as the 16th C, which would date the 1st floor and above being built around that time (the ground floor is a vaulted Middle Ages space).
We racked our brains though: where is the main interior wall that would have originally supported the large ceiling? The beams that remained at their original length suggest a 25x25ft room, a heavy ceiling... but apart from the 'new' partition there is no supporting wall for the original length. We paced the place, we measured, we traipsed up and down the attic stair to compare but no matter how we tried, the direction of the imagined wall would have led right into one of the windows. Then an idea: perhaps the window wasn't there in the first place, and was added ad the same time as the reconstruction we are talking about? We looked at the plan and sure enough there are 5 windows in the facade at that level - 2 and 2 on the outsides with exact distances between them, and a middle one that isn't so exact. Just a theory this (so far) but it is possible that the classicist facade we see now is the result of someone's 'grand design' to modernise their house and bring in more light.....
I am sure we shall get the answers eventually. For now, I am on tenterhooks :-)