I've been told off for proposing the nature preserve idea.
We get told off for lots of things, doesn't mean they are wrong! There seem to be very many reasons why advanced civilisations wouldn't have contacted us. For me the most likely is that we're just not interesting enough, which has elements of most of your pet theories. Let's face it - a civilisation that could get here wouldn't have any designs on our resources. If you could get here you could get to lots of places and the things we have on this planet are all easily synthesisable, at least they would be if you already had interstellar travel. If you have interstellar travel then you have almost certainly gone through some kind of "intelligence explosion" (singularity talk aside for the moment), and the shit we are still dealing with - internecine violence, massive wealth disparity, severe resource over-utilisation, etc. - will make us look little better than rats to a civilisation that has itchy feet. So why come here? Many civilisations might well be sending us messages that, when we are able to interpret them, will suggest we are interesting enough to talk to.
Imagine we are looked upon like a race of rats. If we were the first race of rats that an interstellar culture came across, then we might be interesting to contact/interact with. But what if we were the 382nd? Far away, with nothing particularly interesting to offer, why bother? Like really, why bother?
The main problem with all this sort of discussion is one that is raised by all sorts of deep thinking around culture, beautifully illustrated by Wittgenstein's "If a lion could speak, we would not understand him". We project out what we would do *today* if we had the technology to do X. The key issue is that by the time we develop the technology needed, our worldview will have changed so radically that we have no way to imagine how we would react, and what we would want to do. We talk about "cultural invariants" but what we do is just interpret what we see to align with our current cultural norms. It might work reasonably well with our fellow groups of humans, who all live in remarkably similar circumstances, but it is ludicrous to think that the way we look at the world could be remotely similar to anything that developed on another planet. Or that we are currently at a point in our development that corresponds to a comprehensible level of development in a culture/society in the very near vicinity. Life developing is surely a rare affair, but a comprehensible society developing at exactly the right time in the history of the universe in the near vicinity seems vanishingly unlikely.