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Wrap the whole thing firmly with a towel, something thinner and long, so you get multiple wraps. Then put it in a vice, very gently - just tight enough to hold it. Or let a person hold it.

Then use an impact driver to get it loose. If you use a cordless driver, use quick, short bursts. Like 1/4 second at a time.

Double check whether it's a left- or right-hand screw.

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.

Not laughing - I had a terrible time when I was first trying to get the mounting plate for the camera off of the body of Manfroto Ball mount, requiring copious amounts of WD40, close watching of videos of how it should have worked, and brute force. It now works pretty well, but was a wierd movement intended to stop accidental releases.

If you have a decent camera shop nearby, they might be prepared to look at it (I'm guessing you bought online)

Otherwise, I'd guess you've already tried WD40 anywhere there's a crack that might be relevant (remembering to wipe it all away before going near a camera), and double checking instruction manuals and youtube for videos of mount in action (just in case there is a "trick"). So, ummm, good luck!