Any input is appreciated.
That type of drawer setup can be done with minimum aggravation. It starts off with how you lay out your cabinet. I usually use a sheet of any substrate (MDF, particle board, or plywood) to draw out the cabinet in plan view full size. I draw in everything, at the sizes they are. Doing that allows you to plan the joinery, and deal with spacing and fit.
In that case it would be the whole corner. It would show any verticals, the face frame, and the countertop front edge. With that, you can draw the drawer box, and the front (including the drawer front faces).
If you draw in the cabinet member that the cabinet side drawer slide attaches to, that should be parallel to the other side. Or, If you have a movable wing, or just return strips to mount the slides to, they could be adjustable at the rear. I would recommend using the slides you have in the picture (looks like 100lb full extension, side mounts).
By drawing out the cabinet on a sheet, once you build it, it should fit the drawing perfectly when placed on it. Also, keep in mind that a cabinet will rack when it's in the stage in your photo. A slight bit of rack can make a perfect fit appear out of square.