If you have room. A vertical gable. A piece of mill work coloured material that goes from under the counter to the floor. You will only see the 5/8 edge when your done.

Generally you can custom order windows for Renovations. You give them the total outside dimensions for the brick mold and they build the window to that size. You also tell them the custom jamb depth. I would assume you can have a 60mm depth built. I would call window guys and ask these questions and get a price.

No way. If it’s just drywall backing cut away. I would cut it out without hesitation.