Hello. Thank you for this interesting example. The vertical line in an augmented matrix can be spurred, or it can be made with a tactile strip. You can also use spaced braille dots (456) if you explain in a transcriber's note. I have answered your other questions in the attached file which also includes a simbraille example of Step 3. I would not use a key since there is no reason to follow the side-by-side print layout.