[Turtle91]

Calibre's 'Custom Columns' are for use in managing the books in Calibre, they do not make other apps recognize that non-standard metadata. The metadata is properly stored in the ePub by using the Polish option you mentioned (or the save-to-disk option) but Marvin doesn't know all the possible custom options users could think of, in order to display them.

Marvin's User Collections must be created in Marvin then assign the books to the respective collection.

There was at one time someone working on a link between Calibre and Marvin to automatically do what you would like...and it was quite promising... but then Apple changed their policy on how apps communicate with the outside world?? and it didn't work anymore.

You can manually process your books, though, and it's fairly simple. Just sort Calibre on your custom column, transfer all the books in that column/category to Marvin, then use Marvin's multi-select to add that whole batch of books to the User Collection. Repeat for each category.

Cheers!