
I really thought I was home free once I got them on the piston ok. What is it with these things?
Well, I also bought another set of rings for my old piston just in case. The old piston takes 2mm rings while the new piston takes 1.5mm. Maybe that made the difference, because those went on the old piston just fine, and I installed the piston without incident. I've also had the old 2mm rings off and back on the old piston ring several times without any trouble.

Before installing them, I measured the ring gap to compare with the old rings:

The 0.203mm gauge was pretty tight, but it fit. That's a larger measurement than on that spreadsheet I found earlier, but it is tighter than the old rings.
I got it all put back together and properly torqued, and did a compression test. 101 psi. That's 6 psi less than with the old rings!? I must have a gasket leak, even though I can't seem to find it. I don't know what else it could be.
In the mean time, I found some Magura brake light switches on eBay.

These are used, but they have metal housings and threads. My current ones are all plastic.