I would start with any good degreaser (Simple Green, denatured alcohol, dishwashing liquid) to get the skin oils, popcorn grease, and cheese puff residue off. Then, if you want them shiny, a trip through the tumbler. I suspect having them clean going in would reduce tumble time.

I mentioned pre-cleaning because I was thinking in terms of bead blasting. Bead blasting is good for removing rust, but not grease. If the part is greasy, the beads just embed into the grease and do nothing. The part must be dry before you start blasting.

Here you have a similar situation with dry abrasive (walnut shell, plus what ever polish you add) on greasy (well, kind of) parts. Even if you use Novus 2, I suspect the walnut shell absorbs the liquid, and what you have left is dry abrasive. I was thinking the tumble would have been more effective with de-greased parts.

Having said all that, it's obvious your results speak for themselves. Looking good.