After wrestling with the "matching" solder for Stuller's X-1 white gold for a while longer over the past week, I gave up.
I sawed through the seam (removing most of the X-1 solder), re-tightened the seam, and soldered it with the 20k white gold solder from H&S. Success! The seam still looks a bit sloppy, but it's sturdy and the porosity is gone.
I think I will try some tests on the X-1 scrap using various plumb solders from H&S (20k, 18k if I have it for nickel white, and 14k). I may include the x-1, too, just as a point of interest.
The x-1 itself is polishing up nicely; I hope to have some pictures of it next to both argentium sterling and palladium 950 in the next couple of days, to give a better idea of the color differences.
So, my verdict: it's sure not as white as silver! But it does seem to be whiter than palladium, and with some extra care and a couple of extra annealing steps CAN be used to make a Mobius ring. I'm very pleased about that, and about being able to offer a nickel white gold option for those who really have their hearts set on it- even though I still don't think it's a particularly great idea, personally.