non-gradable: perfect/ fabulous/ fantastic/ awesome/ wonderful/ perfect
Yes, 'very perfect' is clearly wrong, likewise for the others. But 'quite fantastic' is fine for instance, etc. I would put 'fascinating' into that group.
How can I tell? Well, I just ask myself whether if the adjective divide the word in two eg. legal/illegal.
Some more: acceptable, legal, moral, feasible, mortal, original, debatable...
Yes, many work with 'very', but that's the meaning you put in red in your post: " the effect is sometimes to give emphasis rather than to express the degree of the characteristic expressed". Eg. I'm not sure it's very legal. This is a turn of speech; either it's legal, or it's not. Either it's an original painting, or it's not etc.
Some words are too strong to be gradable, like awesome, fantastic etc. How can something be slightly fantastic? Likewise, slightly fascinating sounds wrong.
FRC