Faron or paintguy may have better input, but here's my 2 cents. Before painting latex over oil, you need to use a primer that can coat oil paint and be top-coated with latex. I believe you'll want to stick to an oil primer for this. Once the stripe of primer has dried, you can cover it with your yellow latex. Personally, I think it'd be easier to stick with spraypaint and use delicate surface painters tape & masking paper to cover the areas you don't want yellow.

If you're updating your trim color, white I would gather, I would keep them the same color. If your trim paint is also looking faded or worn, freshly painted risers will likely make that standout more. I would gauge whether you need to paint all woodwork after the risers are painted.

No but I would use a primer just to get good adhesion.

@ Home interiors with ease., thank you! No the stone was painted. That sample I’ve had for a long time since we purchased the stone about 12 yrs ago. I showed it to the painter as an example of how light I wanted my stone painted with a little less yellow.