Looks as if there may be a dimple in the ring to help unscrew it? You could try warming it with a hair dryer to see if it may help or possibly try wd40 (yes I know).
The square spindle is possibly one of those soft metal or split ones which can spread or deform when the grub screw is fully tightened. Remove the grub screws on both handles completely, wrap one of the handles in some rag, place a smooth timber offcut on the door and gently pry off with a 15 to 18in pry bar. It should pull off quite easily.
The rose around the handle has a dimple in it so it may be the type which unscrews (anti clockwise) it it might be possible to just pop it off with a flat screwdriver. That should get you to the screws hidden beneath it
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