Don't get disheartened. There is no problem with the zone on the left side of the masonry wall because it is within 6", (150mm), of the corner and therefore in a safe zone.
Rather than ripping plasterboards off I would chase down the masonry wall on the left side, (inside the safety zone), and then chase across to your 2 fitting positions. If you are going to fill one chase you may as well do the 2 at the same time.
Bear in mind that your cable chases must run in line with the base fittings and not a few inches below as it appears in your original drawing as this would be outside the safety zone because it is not in line with the fitting.