Just wanted to end with an update. After speaking with the folks at Xootr, I sent the bike back (they arranged for a pickup and paid for shipping). The bike tech there says that the chain rub was a manufacturing problem. They switched out the rear end of the bike (and a lot more, it looks like to me-- brake pads, tires at the very least-- possibly wheels?) and returned a swift that looked brand new, and rides better than before.

The original problem is solved, and I am extremely pleased with Xootr's commitment to customer service.