Depending how vivid your scarlet is: You can go with white or cream so it really is the accent wall and nothing is shouting for equal attention You can also go for a muted grey or beige

If you want to highlight the staircase wall then your could use earthy colours such as a yellow ochre! The wall could be enhanced even more by using suitable frames and making a good collage composition for all the photos that you have (I can see many lying on the dining table). This colour would match well with wooden furnitures

Definitely YES!

Unfortunately all your options require custom work: It looks like you need an 18" corner cabinet. You will have to modify the last wall cabinet on the left. As I am eyeballing it, you should be able to cut the last cabinet in the middle and then shorten the left section to fit the space of the wall next to the door trim. It's an option that makes the area looks symmetric. Or you can install shelves over the sink. I would not recommend it because it will take the looks away. And the last option will be to remove the last cabinet, shorten it so it fits over the sink and install a single cabinet where the double one was. This will leave a triangle space behind the corner where the two cabinets will meet. You can block the space under it. Hope this helps!