Everything is working now! Hooray!
Here are a few specifics:
I am printing on cut sheets, not a roll. My understanding of QTR settings is that you use "manual roll (rear)" for feeding sheets through the rear slot, "roll feed" for for feeding roll paper through the rear slot and "sheet feeder" for feeding lighter/thinner sheets through the sheet feeder on top of the printer. This may all look obvious enough, but it was unclear to me at first. I didn't understand what "manual roll (rear)" meant. Could you confirm that I'm understanding these paper settings correctly now? one more thing about paper.
I'd done several nozzle checks before I started printing. They all looked fine.
I hadn't tried printing any other images.
I changed the paper setting to "letter", and now things print fine. No off-center, no "fading off".
The few prints I've done so far look wonderful. They're sharper, more detailed, richer and more three dimensional than anything I've used before. I was never happy with ABW. QTR quad w/Epson inks was a big step up, especially regarding neutrality. Piezo is at least as big a step up again. I'm very pleased with the results.
Thanks again for all your help Dana, especially for a product not quite ready to hit the shelves.