It became about free will because the notion of mental states having an impact on physical processes implies deliberate decision-making and free-will.
" illusion or not, exists, and, since it exists, has a physical presence"
So even illusions have physical presences, in your view?
"How can you deny its existence?"
I just explained.
Your arguments amount to a series of "self-evident" statements, which isn't an argument at all. And frankly the argument is surprising, because I've read upteen Thucy sermons about the importance of being open to other points of view, and how I'm a 'bitter man' and have a personality of 'pure poison' or something because I'm not as open-minded as Thucydides. And here I am and I can't even get a modicum of substantive response here. It's just a series of "how could you" questions, over and over again, combined with "isn't it obvious" statements.
No, the whole point is our understanding of neural processing is not at all obvious, and our conception of the "mind" rests on outdated concepts.
"What the fuck are they and where do they come from then?"
Disjointed neural residue that occur as a result of physical processes. Like the foam on the crest of a wave, or the steam-whistle on a train. Just like how memory and attention are fragmented and do not represent unified processes, so too is "thought". These ideas are just useful (although increasingly not so much) fictions to simplify experience.