The bottom of this ballmount can rotate.
The bracket is attached with a screw to the bottom of the ballmount and requires rotation to unscrew.
The screw is so tight that, even with the ballmount rotation locked, it still just rotates the ballmount.
I can't clamp the sides of the ballmount bottom as it is too fragile.
Surely I am not the first person in the world to have this problem. Is there a trick to getting this loose?
I tried leaving it in the fridge, hoping it would shrink enough to come loose or something but no dice.
Thank you for your time, feel free to laugh at me.
You could try heat cycling it. Heating the screw, not hot but like 60-70°C, then bringing it down to quite cold, say around -15°C, should first push everything outwards then compress the screw. Once that is done a little bit of silicone lube should get through the tiny gaps and help everything move. Just make sure the lube is compatible with your plastics, some plastics are harmed by compounds in lubes.
Past that, try to find out what the cheapest part would be to replace if needed. Make sure to load that part most in opening it up so if it breaks you can replace the cheapest part.
This ended up being the answer. Gentle heatgun and freezer for 10 minutes
Yay!