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.

I do think a medium to dark color looks better, but I don't see how one coat of any color is sufficient unless they apply one coat very carefully, and not thinly. I am skeptical that any contractor is going to do a great job with one coat. The purpose of a second coat is to even out the coverage and cover holidays. Are they going to do one coat and then carefully examine the entire surface for holidays, or are you going to be discovering them weeks after they leave? There is a lot of room between black and white. I find black to be very severe. It seems kind of trendy to me as well. Do you think you would still like it in a couple years? I would consider more moderate dark colors.