I had this great elaborate plan that required rebuilding that whole bunch of plumbing, I’m now not sure why. At last, having gotten it all cleared of dirt and visible, I said “screw it,” made one cut, and just unscrewed the broken valve. First I used the faucet to drain most of the water from the more than 300′ of pipe coming down the ridge, so the whole trench didn’t fill up. I dug a little pit under the valve both to allow it to rotate and to contain what little water came out of the pipe when I cut it, and it looks as though that part will be no bother at all. Don’t know what all the fuss was about.
The riser should be much more of a bother, since it involves crawling around under the cabin and I really hate that.