Well, the last clutch from that male Caramel albino that fathered the pastel earlier in the season has pipped/hatched (mother was a normal het caramel), and even though there were only three eggs, the results are illuminating:
That's right, a perfect little pastel caramel albino, plus another regular caramel albino. To clinch things, the third egg has pipped, and contains yet another pastel! So, in three clutches, this male has sired: 5 caramels, 2 pastels, 3 normal hets, and at least(one of the other caramels bears reexamination)1 pastel caramel!
So it looks like our male was a Pastel Caramel after all, so no "spontaneous mutation" or "new line" pastels, unfortunately. But I am nonetheless relieved to finally know what is actually going on!
Thanks to everyone who very helpfully provided input on this clutch over the last several weeks!