Ok so I have my shiny new mix. If I understand correctly this is the process I need to follow:

Upload my own original sample in the Samples area.
Add additional samples by using “Edit”.
Then upload my track in the Remix submissions area with a trackback to the a Capella I wrote my samples around, and also to the samples I wrote. (Which are really just v.i. Tracks)

Have I understood the process correctly?

Hi Tunes!
If you like to upload a remix you do this in the remix submission area and just add there your own samples as single files (drum file, bass file etc.).
If you have own vocals too for this remix you can upload them
seperately in the accapella section.

Cheers!