Interesting subject and nice essay to present your question as to "does art exist?"
But you start and end with a false premise: subjective phenomena do not exist because cannot be observed objectively. This is not true, and I will show you later why.
But first, a step back to whether art is subjective or objective.
This should be a non-issue. Art is clearly subjective, and anybody who would claim otherwise is absolutely ignorant about art matters. A piece of rock lying on the surface of the moon is not art. It has nothing to do with art. But if a suiseki artist picks it up, puts it on a tray and declares that this is an exceptional suiseki, it instantly becomes art. All he did was to declare it a piece of art and present it as such. A textbook example on what is subjective.
Your next question "Is subjective real?" brings me to an example that can be very persuasive: Fear.
Is fear subjective? Yes. It is clearly in our mind, the next person standing next to us can observe the same thing and have absolutely no fear.
Is it real? It is very real to me, so real that it can stress me to the point of killing me. Psychosomatic diseases can do the same thing. Calling it unreal or non-existent something that can kill you would be foolish.
Subjective phenomena are part of us. Our mind is part of us. It is part of what we do and how we function. And we ARE real. Therefore, everything that's part of us IS real. If observing great art makes me jump up and down in ecstasy, the jumping is real. It is the result of my subjectivity that has an objective manifestation.
So, again, subjective is part of me, and I am real. Therefore, art is part of me, and it is real (has objective manifestations).
The influence of art has interesting implications on bonsai, though. As you've said, depending on who created it, bonsai can be high art or just a knock-off. As Walter said, with painting this is more obvious, the value of the same painting can change from a fortune to almost nothing just as the result of some new research deeming the work a clever copy.
So, clearly art is more than just the work itself. It also has to be the materialization of the original idea , a concept that is independent of the physical work itself. Copies are never looked at as works of art, just good craft. The work itself has to be the original one, created by the artist.
It looks like the formula of art is a function of three elements: f(actual work), f(original idea), f(artist) = work of art. All three elements are necessary for a work of art to be perceived as such. Forgive me the mathematical representation, it looks silly.
When we talk about bonsai exhibits, we often say, "it is all about the tree and not about who created it". But, if we look at the ingredients that makes art an art, we may have a problem with viewing bonsai this way. Because bonsai can change year after year, bonsai can be art today and an artless piece of plant tomorrow. Depending on who maintains it day after day.
The "Last Supper" by Leonardo is a mural that was renovated endlessly, since it is inevitably falling apart. Because it was renovated professionally and with great care, it is still perceived as a great piece of art, although pretty soon none of the paint on the wall may be the original one. The renovators painted practically all the painting.
I guess bonsai can be very similar. If maintained with skill year after year, it can remain a great piece of art. If changed by an artist, it can become a different work of art. But it can also degrade into nothing. It all depends on who is working on it.