We used 1x6 for base, crown and tops of doors/windows. (Beams/crown in this photo are cedar.) 1x4 on sides of windows. Ours is stained. Dust isn't that bad, no more than regular trim. I wipe it with a Lysol, etc wipe or damp sponge once a year. Been in house 4 years. fwiw, we got the idea from a neighborhood we toured that is done in Craftsman style. All their houses use plain boards for trim/painted. Here's a shot of the breakfast that shows it pretty well. I don't normally leave photos large, but will so you can see it better:

I'm assuming the top edge of the tile on the rest of the backsplash is ok........and that the gap appears only under the window? My previous home had a similar window and the installer covered the 'sill' (which was drywall) and the sides of the window opening with the same material as the backsplash. IOW, the granite wrapped around the lower section of the window so that the backspash was the same height across the whole length of cabinets. I can see why the installer wants to use bullnose since this will give a more professional finish. Would he use bullnose across the full width of the backsplash?

I've never seen an installation detail showing the wall panel overlapping the flange. That would require shimming the panel after the base in in place and if the wall panel gets too close to the shower base water can be drawn up into the panel. In your case the mistake was cutting the wall panel so high above the pan flange creating the grouting problem bry mentioned. You might be able to use a Z flashing or larger tiles (see below). You could cut 1/2 or 3/8 x 12x 12 marble tiles in half and place them horizontally in a running bond pattern solving the bottom edge issue (see below). You could add a narrow 1/2" thick strip of wood at the bottom of the wall board to reduce the gap and install a Kerdi membrane over the entire wall. Its difficult to tell from your photo how close the shower base flange is to the studs but assuming it is no farther than 1/8" from them here is the standard tub and shower base detail for tile or solid surface panels: For an excessive gap a Z type flashing might be appropriate: Be aware that a silicone sealant joint should be deeper than it is tall so it forms an hour glass shape in section allowing the silicone to stretch without losing adhesion. This is easier to achieve with 1/2" or 3/8" thick marble tile than with eased edge ceramic tile.

@zealart - You are correct. As President Reagan said, trust but verify. I just finished a remodel including gutted bathrooms with new showers installed. I spent a lot of time on the Board reading about shower construction and horror stories. I followed links posted regarding shower construction. Much of it went over my head but I retained enough information so that I was able to ask reasonably questions regarding the materials used and how my shower was being constructed. I also checked out work done (including showers) by my designer and GC. All efforts put into researching and learning as much as possible will get you the best possible end product. And not just in terms of actual construction - but taking the time to read copiously enabled me to learn about how I wanted my space to function and what products or design elements could get me the results I wanted in both design and functionality. I did have a great designer but it really is more of a partnership as I needed to tell her how I wanted my space to work - and sometimes there was stuff I read about on this forum that I was able to tell her about :-) - like my toe kick step drawers. I also live in an area in which stuff is RIGOROUSLY inspected at every step by the City. My shower pan had to pass a 24 hour flood test for starters. I took pictures of my showers at various stages of construction which show the waterproofing and materials used. I do know that my shower is sloped - I can even see where it slopes even more markedly by the drain. There is no puddle or even small remnant of water left when I finish. And I doubt any slime or mold is growing anywhere either since my automatic fan runs until it registers a low humidity in the bathroom. FWIW, my curbs appear to be some kind of concrete and the curb is solid surface - constructed from a remnant of my counter and my bench is constructed of the same material. From everything I have read, solid surface for curbs and horizontal surfaces is recommended over tiles.