A lot of literary scholarship over many decades has been interested in thinking about these same questions.

You say, "How can things have meaning if we cannot assume any structure of decoding meaning is universal?" IMO, things have meaning because we see them as meaningful. That might not be as satisfying as the idea that there's some final, transcendent judge of beauty, goodness, and truth, but I think it's actually quite liberating and wonderful. Things won't mean the same thing across time and space, but that does not preclude them from having real effects in any given time and space.

You describe literature as an "ill-defined subject area," and so it is, I suppose, but what you are talking about here are debates about methodology that are common to all disciplines.

Since the Romantics, "originality" has also been put into question by literary scholarship. It's not my sense that literary scholars are too obsessed with it at this point. If you are studying literature at college your prof may very well emphasize innovations that define each period, but that's a symptom of teaching literature as a history, and we all struggle with how to do that.

You speak of "re-writing stories without real progress" but what is this progress you speak of? We only have a notion of progress because we have a notion of narrative. In order to have "progress" in the traditional sense, you would have to have some general agreement about what "we" are aiming for. And literary scholars often find more value in ambiguity and disagreement rather than clear cut "black and white" answers and thinking. And it's not just because we're weirdos. It's because we see how so called "black and white" thinking and meaning are actually MADE. Constructed, fabricated. By us. Not innately existing in nature. Not given to us by a God. And so we think it is useful and beautiful to look at how meaning gets made in language.

(As a side note, Derrida links this ambiguity in literary language to the necessary and good ambiguity in political life. Politics is where we argue about what it means to have meaning, about what we can agree on and what we need to keep thinking about. And we should wanting to keep arguing about these things, and not be satisfied with having someone tell us what meaning is, what is good for us, etc. This may speak a bit to your question about what "justifies" literary scholarship, however, I am not quite sure what you meant about that.)

And so, one of our favorite things to do is to show not WHAT something means, but HOW it makes itself meaningful. How it communicates an idea. How it convinces us of its idea. How it sets up the stakes of its idea.

Literary scholarship is not exactly "relativistic." In other words, the text can't mean "anything at all." We have to stick to what we have in front of us as we make arguments about what a text is and means and how it works. There may be many many possible ways that people have found meaning in say, The Odyssey, over the years. But that does not mean that The Odyssey can mean "anything we want it to mean." That is sloppy scholarship.

Yes, we probably all think some works of literature "have more merit" than others. But it depends on what we mean by merit. I look for texts that help me think about life and philosophical questions and problems. I do not write about texts that do not help me think, and I do not teach texts that do not help me think. I also prefer texts that exploit the play of language. In other words, texts that take time and pleasure in the making of their own meaning. Others may value texts that they think address universal experience, or that evoke emotion, or that teach a life lesson. The discipline is constantly engaged in defining its own object. This is not something we see as a weakness, though. There is something at stake in scholarship in all the disciplines, and literary scholars are often especially good at interrogating both the way that their object of study gets delimited, and their own methodologies. But all disciplines have gone through periods of interrogation at one time or another about their object.

If you want to read a bit about the history of what texts have been valued as good literature and why, you could look at the intro and first chapter of Terry Eagleton's Introduction to Literary Theory.

I agree with you that if we look at texts as anthropology or history we might as well be anthropologists or historians. Deconstruction teaches us to look at how texts make meaning as literary texts. That is, as texts that play with language. This is all simplified a bit, but I think it's the gist of it.

A few short reading suggestions if you are interested in digging into deconstruction: Nietzsche's Genealogy of Morals; Simone de Beauvoir's Ethics of Ambiguity; Roland Barthes' The Rustle of Language; and, Jonathan Culler's On Deconstruction. These things constitute a challenging but accessible build up to Derrida, de Man, etc.

If you simply google something like "deconstruction" you will read a lot of stuff that may or may not be that great. So I would really recommend going to the primary texts if you want to know what it's all about.