I have been continuing my tests. Today I did a series of systematic ( if not very scientific ) tests with Adorama brand VC paper. It behaves similarly to MGIV in alkaline solution, in that it self-limits ( exhausts, I think ) and the effect varies with pH. It tops out at a lower pH and does not get as dark as MGIV, but it still gets very dark. MGIV achieves a darker and more neutral color at a higher pH where Adorama no longer gets darker. The Adorama color is a rather ugly greenish dark brown. Today I ran a series of tests at different temperatures between 60 and 90 deg. F, each with 6 different pH levels. Like MGIV the development appeared complete at all temperatures between about 15 and 20 seconds. There was a subtle difference in the final color at different temperatures, but you have to look hard to see it.
Anyway, the reason I came back to this thread is that I just came across this "paper test" for whether xtol is still good. I see xtol is pH 8.2, which isn't that high, but it makes me a little skeptical about trusting this test!