SF and I worked through a lot of different approaches to confirming that no useful data was deleted during our cleanup. We have plausible explanations for all but 32 of the orphaned owners; and we have identified about ten titles which were deleted by editors during the summer work period; these must have been purposefully deleted around the time they were created -- they never got saved into a backup -- so they must have been erroneously entered. I think these are the plausible explanation for the orphaned owners.

In the process I added a new titles as seller field to the owners table, and we confirmed the consistency of data in lots of other respects, so we're looking good.