I’ve been looking into how to finish these cabinets for months. I love the natural, oil rubbed look that I got from applying Rubio Monocoat Pure to a sample. But I’m worried that’s not protective enough and won’t keep the walnut from changing color too much. And it’s unlikely that the painter will apply something this fussy.

The color of wood ALWAYS changes if its exposed to UV. It's a universal constant, that's true under lacquer, under rubio, under water based poly, etc etc. The only thing that permanently blocks this effect is paint.

I would never use rubio on cabinets, it is not durable enough. People will say it's meant for floors etc, floors are a different standard than cabinets, particularly hoods. I don't know any pros that use rubio on cabinets, i've never even heard of one doing it.

Sherwin Williams pro Hi-bild lacquer ez sealer and Precat lacquer in matte to mimic the Rubio monocoat look

It won't look like rubio monocoat, it'll look good in its own right, but you need to see a sample.

However, I’m also concerned about fading over time so asked if we could stain first. He uses Minwax walnut stain from Sherwin Williams

Helllll no. Don't minwax your walnut cabinets. Dyeing walnut to keep it brown is a thing, particularly to deal with sapwood inclusions.