The picture with the flowers on the smaller wall look perfect. And in the last picture stack those 2 pieces. One where you are holding it and the other about 2-3" higher.

Just keep the painting or mirror within the width of the headboard & pick a piece that is not very high or deep. Good suggestion with the mirror.

Hello - as a I have always used for my clients projects 15-30cm beyond either side of the window and 12-15cm above the window frame. This may vary depending on the window and what's around the frame and if there is suitable fixing when installing into the plaster or wall.

Instead of doing something like a paint color, why not expand into doing something a little more bold and daring like wood planking or grass cloth, etc.? This will not only help accent your "neutral," kitchen wall color, but will also allow you to continue flow throughout the space and give you a more well rounded and grounded space. I would heavily consider doing something that adds not only texture but depth to the space, paint color is great, but accent walls are a thing of the past! Give your kitchen the POP it deserves and your space will be transformed.