So here are some more tests
For the 1000V input, AC test up to 2kVrms, there is a limiting device that trips at around 1.9kVrms:
For the 2000V input, AC test up to 4kVrms, no problems, although this is with the 760k resistor up front instead of the original PTC:
For the 2000V input, AC test up to 5kVrms, no problem, current meter reads 250uA as expected, I don't really see the point in going further, 5kVrms is 7kV peak.