Once approved by ACX, do I submit one chapter at a time … or do I wait until entire book has been read and THEN upload ALL the chapters?

They publish a Help Center with a lot more detailed info.

One of the problems you may have noticed is they go into Great Detail about the color of shirt you should wear when you appear on television talk shows to promote your audiobook, and not such great detail about the actual process of producing the work.

There is also a less obvious problem as well. This is a forum, users helping each other, and what generally happens after we get a poster/reader working, is they vanish never to be seen again. So we’re always missing the Last Mile to address your exact question.