Can literature "tell the truth" better than other Arts or Areas of Knowledge?

My answer to this is a firm "Yes". Novels, for example, "tell the truth" better than any other written material, with the exception things like diaries and letters, unless you think of the relevant passages of diaries and letters as though they were mini-novels. But diaries and letters are no better at telling the truth in the appropriate sense than the skills of their authors. What sense is the sense in which novels (or more generally imaginative writing) can "tell the truth" better than any other "Areas of Knowledge", as you call them? (I imagine that you might have the sciences in mind.) The sense is one in which telling the truth has to do with getting the details of a description absolutely right, and getting the overal balance and colour and mood of what one is describing absolutely right. Here psychology for example (which might be thought to "tell the truth" better than the novel) is no better than the sensibility (the eighteenth and nineteenth century word) of the individual working...
Art

Can you name an attribute such that all the paintings which have this attribute are good paintings?

You might think that translucency is a good thing in a watercolor, but not in gouache. Versimilitude might be good in a portrait, but not in an expressionist landscape. And so on. On the other hand there is a logical (or with a stretch a "metaphysical") attribute that all good paintings have. They meet the criteria for excellence in paintings of that type. Helen Knight on the use of "good" in aesthetic connections is brilliant on this subject.
Art

I recently read a text by Michael Fried, "Art and Objecthood," in which he rather vehemently opposes "theatricality," suggesting that it is the antithesis of modernist art (or the "art of our time"). He never really seems to explain why he is so opposed to it, but he uses extremely aggressive language (to my mind), talking about the perversion of art sensibilities, the corruption of art by theatricality, the "problem" with Minimalism (without saying what kind of problem he's talking about), etc. I was wondering if someone could shed some light on what is so terrible about theatricality, such as to merit such strong language.

I haven't read the Fried piece, but I do see that there is an obvious objection to "theatricality". The objection is that it is phony and therefore cheap. Or one could go further. One form of phoniness is insincerity, another is sentimentality. Consider for example the difference between Rembrandt's sketch of "A Young Woman Sleeping", in the British Museum, a modest but tender sketch of his girlfriend, and Käthe Kollwitz's brilliant but melodramatic sketches of mothers and children. I am not sure whether the objection is completely overwhelming, though. Herman Hesse's Glass Bead Game , for example, is theatrical.

Do most aesthetic theorists in philosophy think that things beside art can be aesthetic (such as everyday life when not presented with art)? Or is that something only a few philosophers advocate (such as Dewey and Wittgenstein)?

Most aestheticians make the distinction between aesthetics and philosophy of art, with "aesthetics" being the wider term and "philosophy of art" the narrower one. "Philosophy of art" is only the philosophy of works of art or art objects as they are unappealingly called these days. In other words, these philosophers accept that it is not only works of art to which the terms of aesthetic appraisal apply, such as "attractive", "unattractive", "lovely", not lovely", "unlovely", "majestic", "grubby", "oily", and on and on, without end. They also apply to the human face and the human form, to nature and parts of nature, including natural landscapes, the sea, etc. There is practically no word, I believe, that cannot one way or another be used as a term of aesthetic appraisal. The aesthetic is everywhere; a happy thought.
Art

Let's agree that something is art if the art world views it as art. Many famous painters were refused exhibition for years, only to have their rejected works considered masterpieces at a later date. Others were considered great artists, only to be virtually forgotten. Operas go in and out of fashion. Same with literary works. Does this mean that the same works can be art at some points in time but not art at other points in time?

"Let's agree that something is art if the art world views it as art." I think that we shouldn't agree to this proposition, precisely because it does seem that it has the relativistic implications you describe. So I take your question to be, "Does the institutional theory of art, e.g. George Dickie's (the theory that something becomes art if and only if the 'artworld' gives it this status) have the implication that something can be a work of art at one time and then not be at another?" The answer the institutional theory gives is that it is the totality of all the little artworlds at particular times and places inclusion in which at any time makes something art. Does this answer work? Does it make indeterminate today whether this is a work of art, as whether it is seems to depend on the possible response of some future art scene. Some ugly Greek pot is displayed in a museum by lovers of Culture today, so it was art even if the people who saw it at the time it was made dismissed it as crassly...

It has been suggested that the practice of Bonsai is an expression of animal chauvinism and does great harm to a tree by 'stunting' it. But aren't trees not sentient beings, and therefore the excising of branches, shoots and roots such that the tree thrives albeit substantially smaller than its genetic potential, is no different to the continual loss of roots, shoots and branches that occur under natural conditions?

There is the fact that it is possible to treat something badly or to damage it, whether or not it is sentient. A pair of shoes can be badly or well looked after, and it is wrong not to look after a good pair of shoes properly. A living thing like a hedge can be properly maintained or attended to, but something more is involved. Sometimes, it seems to me, the aesthetics of the "art object" (horrible phrase) can reflect what it actually is. So it is natural to trim a box hedge in one way, and a mixed hedge in another way, perhaps a less formal and more undulating, the shapes reflecting the different kinds of growth - hawthorn, or privet or beech. (A hedge that is full of straggly and unwanted sycamore looks awful, and the holes soon begin to show.) Pugs and poodles may be perfectly happy qua sentient beings, though often there are specific health problems with specific breeds, but it is perfectly coherent to object to the whole process of breeding and thinking of living things as being objects for our use ...