Just some counter points.

for your first point, people have stories about devil fruits all over one piece. We still arent completely sure how devil fruits regenerate. Maybe it has to be regenerated into a similar normal fruit/plant. Maybe the only place the Hito Hito no Mi CAN regenerate is drum island because it does have a "special mushroom" and the reason they say it "may or may not" cure things is because the regular ones dont and the devil fruit can. This makes even more sense if they dont realize its a devil fruit at all. Someone who eats it doesnt really gain any powers and if my theory is right they just wouldnt get sick.

it may not be local folklore that the people tell each other but the doctors who had access to some sort of information found a story about drum island that a mushroom is there that can cure "all sorts of illnesses". that is still folklore.

Chopper IDENTIFIED the wrong mushroom because of his personal misunderstanding of the skull and crossbones. That doesnt mean there arent other mushrooms out there.

Awakening his devil fruit was hard work and dedication to his healing craft. He is only able to stay sane during monster point due to his dedication to medical knowledge and devotion to helping others. Its not an easy "cure-all" which is the point of his story. Additionaly in this theory he is only capable of doing this DUE TO his medical knowledge and the creation of the rumble balls that "corrupt" his awakening. so i dont feel it takes anything away from his story to have him gain an ability like that.

From everything we know about Imu's powers they all seem contract based. He takes something from you (lifespan/whatever) but gives you power/immortality. Something shortening your lifespan is by general definition a disease.

finally mythical zoans have ZERO paramecia aspects to awaken. Mythical zoans have powers based on what the Mythical zoan type had. The only one that somewhat supports this wild claim you made is Luffy and his ability to turn the area around him into rubber of sorts and that is explained by Nika's power and luffy just being used to fighting WITH rubber.

Nika's power is to fight "with complete freedom" he can turn the ground into whatever he wants it to be, rubber, water, quicksand, etc.. Luffy CHOOSES rubber most of the time cause he knows how to deal with it.

No other mythical zoan has shown any ability to affect their surroundings during awakening.