Is a wooden buck necessary when installing windows into full width brick?
Windows are installed directly to structural masonry openings in Europe all the time. Since you already have a sloping sill, you're good there. You'll need to have the windows fabricated for a very small gap between the windows and the brick. Then screw the windows into the brick with shims and masonry screws, and fill the gap with backer rod and caulk on the outside (100% silicone, butyl, or polyurethane--no latex/acrylic!) Spray foam the remainder of the gap from the inside, and finish the interior however is most appropriate--likely with plaster.

Don't worry about your formstone cladding. It won't hurt anything, especially if the brick sill extends beyond the formstone, allowing water to fall down through the air rather than dribbling down the side.