The baking soda turning yellow over time issue: I haven't found it much an issue. It could be that varnish prevents or slows down the effect. I have always applied a good layer (usually several) of matt varnish. Yeah, I said gloss varnish above, but that was wrong. Use sprayed matt varnish. My snowy terrain hasn't turned yellow much over some two years - except few little spots. I suspect these parts didn't catch that good varnish. I tended to it by applying a little bit more PVA/soda atop the slightly yellowish areas! About 5 minutes work, now as good as new.
I haven't used white paint in my mix, so I can't speak for or against it. I suspect it may take away some of the powdery effect. But my final resolution, if my snow terrain would suddenly start turning yellow, would be to dab white paint over it. Doesn't look like it ever has to come to that.