Coke,
I had always been told that not using the driver brakes on long descending grades was to prevent the driver tires from overheating and possibly slipping on the centers. Your other points all sound like valid ones as well.
I was interested in your comment about changing to ET brakes in the 1940's. I have seen lots of photos of C's and K's with a retainer valve on the right front water leg of the tender, but there has often been a second cock located there as well which I have never been able to figure out. Any ideas?
Thanks,
Michael Allen