All excellent points, and using stones to lap stones is Old School and very rewarding. When using any of the Atoma or other diamond products, they will mellow out over time, become finer in feel and cutting action and can become excellent media to raise a slurry for final hoing on finishing stones before any polishing stages. So at some point the #400 will begin to act like an #600 while the #600 will break in and begin to act like a finer grit also.
Alex