Well after getting all fired up this morning about my supplier, everything worked out okay. Better than okay, actually... fantastic!
At first, I didn't think things were going to go very well... I got transferred to three different people. In the end, that worked out great, though, because I talked to the actual person who packed my order, who went to the actual bin the fiber is stored in. It seems that one of their sticky labels "migrated" from one of their giant bags to another. They were super-grateful that I pointed this out to them, and apologetic about the problem.
They not only are sending me the correct fiber, they are sending it Priority Mail instead of my usual snail-pace UPS. I don't have to ship back the incorrect fiber. Which is a good thing, since I'd dyed up a third of it already.
The only bad part about the whole thing was that I was so set for a confrontational, bad phone call, my adrenaline was still pumping when I got off the phone. I probably sounded like a bibbling idiot. :D
Anyway... the "mystery fiber" is regular merino/tencel.... NOT superwash. Hewitt's Woods (posted to the shop yesterday, and taken down briefly) has been correctly labeled in the shop and reposted. The poor batch of "Wicked Candy" roving (based on one of my sock yarn colorways) is bound for the "oops" collection, since it's pretty felty.
I would have sworn this had nylon in it, because I don't get that lovely streakiness in my superwash merino/tencel like I did in Hewitt's Woods. My guess is that they don't blend the two fibers as much when it's straight merino, for whatever reason. But watch my tencel listings carefully for the next dozen or so listings, as I wouldn't want one of y'all to end up with the wrong type of fiber.