Have a return made that goes the same height as the marble under the counter, then paint the rest of the window space agreeable grey. I do not think it will look weird nor do I think it is too late to do that work unless there are truly no remnants of your marble anywhere. If you don't have remnants then just paint the whole space agreeable grey and see if it needs anything else. I bet it will look fine.

I think it will look ok once everything is done. You could remove the valance. You could hang a sign or interesting art in that space. It's not ideal, but once it's all painted & complete I think it will be fine.

The corner of the backsplash has been smoothed. If we did the return, I am worried about the joint from the backsplash to the return and how it will look when you view it head on. If it was planned, we could have had the backsplash a tad longer to miter the return. The fabricator and contractor both say don't do it. Ugggh.

We were hoping for lighting in the valance, but we might have to forego it....

not meaning to be harsh...but the window looks old. [it looks like some we replaced] are you due for a window replacement.? But the black color is fine if it's not time to replace....[ currently seeing black details in kitchens] . Use of caulk in the small gap....and you can get gray caulk....then paint the drywall and return around the window one or two shades of gray up from your counter and backsplash color. Then proceed with all your details re valance...window treatment..etc. You will not be concerned once the surface is even, gaps minimized, color blends with good primer and paint, and everything gets done. I would vote for window treatment[ nice solar shade or something]

I think all it needs is a light, but that is just to make the valance make sense. The window casing and backsplash look nice and minimal. That will be fine. I do think you could paint the window area the same as the rest of the walls, but you may feel differently about the white once the light is up.

@herbflavor - ha! brand new impact windows! I didn't send the best pics. We went bronze because my whole back wall (25'+) are sliders. I figured the look would blend a bit more once we finished painting and caulking. I am just disappointed we didn't plan it right. I feel like there is a solution I just can't find. Thanks for your help!!

If you can't do a pendant above the sink, what about a window treatment?

How about getting rid of the valance and/or putting in a sconce over the sink? I agree not having a return is better than a return with a weird forward facing raw edge.

Do something out of the box... paint the walls red or turquoise or yellow just around that area and put up some interesting wall pieces - a large fork sculpture or a cool clock...just have some fun with it...