I DO like that russet harness on Polk! It looks great!
This is quite a challenging situation, however. Logically if you are going to mix finishes, the seat box is painted and the undercarriage is stained, but I know what you are saying about stripping wheels. Did it once. It is a WHOLE lot easier and worth it to get new wheels than to remove paint for staining, especially if there is primer under the paint!
I guess I am leaning towards painting the entire thing except for the seat panels and backrest. If you do that, paint the seat board, too, including the routered reveal "under" the seat panels. You also have your wicker that would tie in nicely as well. Unless there is a way to take those seat panels off, ALWAYS stain and varnish first! Then tape and mask it off and paint the rest.
Or paint the wheels and shafts, and stain the entire box, including floor boards and dash. We do have a client that used her existing wheels that were painted. We made a new box and she stained the box ebony and painted the wheels/metal black for her grulla POA. She has a black harness and black trim. It looks pretty good, but it hasn't been out yet for the judges to see. She eventually wants to get new wheels, and have the whole thing be stained.
Check your cherry stain on a scrap with the blue paint to see that it doesn't "fight". One sounds "warm" and the other "cold". I have to redo the upholstery on my pony cart because it is too cold, and my stain is a beautiful warm chestnut. It fights each other. Blech! I am going with a warm tan (if I ever show the pony again....)
Really, what I would do is take this photo and "color" it, either with your computer, or colored pencils. Make a number of copies so that you can change the look.
Routering the top edges of the floor boards would "soften" the look a little. Don't router the bottom edges next to the iron. The dash could use some shaping and a brass rein rail, too. Remember, your shaft trim should be russet as well with that harness. That will also tie in your stain.
Even though Greenall says that upholstery should be gray or tan, I think I would go with a blue to match your paint. You will add too many "colors" otherwise especially with your "colorful" horse. (Actually, I think the gray or tan part refers to Bedford Cord, so if you use something else, it shouldn't matter.) Try to get your upholstery first. It is easier to "match" paint and stain than find the right upholstery! And I would get your striping similar to your stain. Stick to the two "colors". However, the outer band of the hubs on the wheels should be black.
Caveat, this is only my semi-educated opinion. If you really want a good OFFICIAL opinion, I would contact John Greenall and ask if you can send him photos. I would hate for you to put all that work into the vehicle just to find out at a show that I was way off base!
If you ever get him as a judge, he is a stickler! We have a friend that has a wonderful russet turnout with her buckskin Morgan. John wouldn't use her at all in Turnout because her lamps were painted brown instead of black, and lamps historically were never painted brown. (He told her this.)
Myrna