I agree that not everyone agrees on art or morality. I also agree that in the middle of the bell curve, questions of art or morality are pretty obvious. Beatles are obviously good art (even though I’m not a big fan of Beatles). Killing innocents is obviously a terrible moral choice (even if I was a genocidal maniac).
My point is that, when presented with the really interesting questions, when looking at the very fringe questions, art and morality are not similar. Fringe, extreme, disagreeable art - it could be considered art by joe, and garbage by jill, as you point out. However, fringe, extreme, disagreeable morality - this is not dependent on subjective consideration, but rather is a sum function of harm vs benefit, like I explained above.
The difficulty in saying X is art or is not art lies in truly subjective circumstances, in the same way that some people might like milk while others don’t.
The difficulty in saying Y is moral or not moral does not lie in anything subjective, the difficulty here lies in doing the exact computation, because in difficult moral question it is difficult to fully quantify every harm and every benefit and to compute the sum. But that’s a matter of technique, not subjectivity.
At least, that’s the way I see it.
edit: I didn’t read your entire essay, but skimming down to the conclusion, you seem to agree on me on the point of ambiguity in art: “it becomes clear that the audience is the best indicator of the value of art.”
Please correct me if that’s in error