Second question: is there a good CSV import reference page for errors? I recently had an error “invalid customform reference key 166” and have no idea what’s wrong with it.

I agree that these messages should be improved - reference key 166 refers to the internal ID of the custom form used in your import. I believe it is in step 2 of the import process, you can select additional options to expand for options, included custom form. Try using a different form. Also, check your mapping to make sure that you didn't map an internal ID of a record to the custom form field. Another to check is the list of custom forms in your account. You can show internal IDs and see if #166 is inactive or unavailable to the role that you were using in your import.