I am not familiar with the intricacies of the K1100 engine, but have owned 5 bricks of which 2 were 16v K100's.
In my experience, the normal temperature reading on the gauge is about 3/8 of the full scale, so I suspect what you see coasting downhill is about right. The higher readings are probably the result of reduced heat transfer from the radiator.
I had a similar problem with my first 16v K100 that turned out to be caused by a lot of minerals in the cooling system with dirt and insects blocking the airflow through the fins. It took two cooling system flushes to get the minerals out of the system. Then I had to remove and soak the radiator in vinegar to clean out the dirt and insects from the fins. After that, the temperature stayed a lot more constant and didn't rise up beyond 3/4 unless I was in stop and go traffic on a hot day.
The cooling system on the 16v bikes seems to be marginal and needs to be clean inside and out to cool efficiently. You also might want to consider adding a manual fan switch.
Last, before you spend the money to replace the thermostat, you want to pull it out and do a test to see how it is operating.
Not knowing what seat you have, my only advice is to take it to an upholsterer and explain what you need.
Your fuel consumption doesn't seem to be bad, I would suspect that the soot you see on the pipe may be the normal amount that will collect if you are riding in traffic with a cold engine in the morning. There is a reason that BMW advises riding off as soon as the engine is started. The engine is tuned to run rich when cold.
My 16v engines both loved their oil and used a liter of synthetic 10w40 between 1000km and 1300km. Switching to Castrol GTX 10w50 slowed the oil usage to 1 liter per 1600km. Changing the valve stem seals on my first bike only slowed the consumption a small amount, almost unmeasurable.
It may also be that your oil control rings are stuck, especially if the bike has been idle for an extended time. Running some Marvel Mystery Oil in the crankcase and in the fuel may help loosen them up. It helped a bit with my bikes. It took about 2500km to get them loosened up after sitting for 5-6 years.
I don't know how tall you are, but my helmet gets a lot of turbulence noise off the windshield on all my Bricks. Adding 6-7cm can help, but I got the best results by cutting the windshield down to put the turbulence at shoulder level allowing my helmet to ride in clean air.
On the suspension, check Race Tech or Progressive for fork springs. I put a set of the Race Tech springs on my K75RT and the ride improved a lot. In the rear, I would just get a good aftermarket shock fitted to your weight. Many of us have had good results with YSS rear shocks.
When you had starting problems, you weren't trying to start with the side stand down? I have done that a few times and it causes the engine to flood and be very hard to start. When I have that problem, I will unplug the tank connector to allow the engine to crank without the fuel pump running for a few seconds to blow the excess fuel out of the cylinders. Then plug the tank back in and the engine should start normally.
