Here's an update. It looks like this will have a happy ending. Elbert at Juki looked at my pictures and saw that the feed teeth were unlevel. He had me call on skype. DH and I turned the machine onto its back and, with great trepidation, took off the bottom plate. He helped locate the right screw and make an adjustment. (I was barely breathing.) Then I put it back together and did some test sewing. It was greatly improved, and any marks left rubbed away with my finger, unlike before. The moda marble has kind of a soft nap to it.
I am very grateful for another referral, McGargantuette, and for the feedback from Quilts4Charity - you are the first person who's said you'd seen this, too.
I hope I haven't deterred anyone from buying one of these. Once you sew on one, you'll see what why people love them. It was a "love at first sight" purchase for me, and I am a serious ditherer and analyzer.
hugs,
Charlotte