[stumped]

Maybe you can find an example of a multi author public domain or free book which goes wrong in your workflow, so that others can try to replicate exactly what is happening

It may also be necessary at various steps to look inside such books with an editor to see exactly what metadata is in the book, as opposed to other places in the database

I don't use sources like gutenberg, maybe someone who does can suggest some good examples

I am also puzzled by how you Create a book in calibre. Do you mean create a database placeholder entry or do you mean create an actual epub? Book from scratch. There is no way to directly type in a mobi format book, mobi has to be the output of a conversion, or the result of an add book operation on a purchased or downloaded mobi file?

If you can give us an idiots guide to create a 2 author book, as you do it, we can try to emulate your exact process