Evening,
I am an engineer and like to shoot architecture. I have a K5 and a 28 shift as do you. It's bulky, and certainly not very portable. The K-01 is smaller but still similar in size to the K5 - nor is it really pocketable either. I also have a Q. The Q's size is wonderful and with its smaller sensor it makes a wonderful camera platform for telephoto work - which is not landscapes. A smaller sensor is not really conducive to wide angle shooting.
So, what to do? Several years ago I also picked up a Panasonic LX3, with a f2 Leica zoom lens (28mm to 60mm equiv) and manual controls. I found it nearly perfect. I liked the short zoom, sized right for carrying around - essentially pocketable (coat, jacket, but not Levis). The down size was the sensor size - which did have some noise in terms of ambient low light shooting - and it topped out at ISO 3200. But, I really liked the concept and the execution is nearly perfect. By the way, I still have it - even though my son took it away to school - he brought it back when he graduated.
Where am I going with all of this? The right sensor size for shooting architecture is really full frame, which is not really pocketable either (even with the new Sony RX1 (priced at $2,800)). The next best option is as you have observed - an APS-C sensor - the K5/K-01. But it too is not sufficiently portable. There is the 4:3 (Olympus and Panasonic) -the EPL-1's etc. Still a bit bulky for everyday. That takes us to the RX100 as others have recommended. Larger sensor than the LX3 - should be better with the noise (like the Sony sensor in the K5). It does have a fixed lens, but at 28-100mm. I have found my LX3 easy to shoot and it was the first camera to have a wide fixed lens and it does work very well. I think your concerns are well placed, but I also think that the RX100 will fit your purposes and needs very well. Photography is a compromise - and I think that the Sony is a reasonable comprise for your intentions.
I would also suggest stitching, for wider shots. Microsoft ICE (free) is very good at stitching the results together - and the price is right.