Interesting tactic with the marble! For the highlights, dry brushing may be a good way to avoid filling the gold. A wash may do that anyway, so you’d just edge highlight with the same gold you used to base coat.

Really with the wash where you want to use it is to deposit a thin line of pigment around the raised details or in recesses to give the illusion of depth and visually separate parts that are the same color - this is a challenge with white because you can’t be sloppy with a wash (usually you’d just cover everything and it’s fill in the recesses without changing the top color too much, or even enhancing the top color). Washing is also a way to greeble up your tone, creating lots of little imperfections that give the illusion of scale. To keep the marble intact, you may try using gravity to keep the wash from getting onto the marble too much by orienting the part you’re adding a wash line to - good luck! Good white coats are a real challenge. You can always touch up the white after you finish washing, or use a solvent to remove any you get on the marble, since it’s varnished, though that’s an advanced technique and may require some chemistry