I realized that I on my first attempt, I set everything to be an update key (bad idea) and on my second attempt, I set only the name to be the update key (which also errored out) causing my frustration and my previous posts. Then I went for a walk, came back and realized that I'd left in all of the previous test duplicates. Deleted all but one, tried again, updated fine. Going to try with Contacts but I imagine it should work.
@Andy, yeah, I had a CompanyName column and mapped that to a path Company.Name.