People have said that for decades, but I think it's been pretty thoroughly refuted. Here are some good links:

Consciousness is irrelevant to Quantum Mechanics

While in the future we may discover quantum effects that bear distinctively on conscious cognition ‘as such,’ we do not have such evidence today.

Sorry, I'm not sure I understand the question. I agree with the second part, though. It has a lot of attributes that make it difficult to describe, and it's something we give great importance to.

Edit: to try to address the question, I think human behavior is the best evidence. We demonstrate awareness through our actions. There are other terms we can use to describe these traits, though.

Couldn't any computer which monitors its own state be reasonably described as self-aware? I feel like a more precise definition would incorporate something like sentience.

I don't think we lack evidence for what it is, so much as we simply use the term to encapsulate a great many concepts. The lack of an agreeable definition is more a failing of language than a result of the mind being "mythical", as you said.

I fully agree with what you're saying. In philosophy it's often described as something physical, and so it stands to reason that it would leave physical evidence. It's difficult to observe the brain while it's still working, but that doesn't make the mind fundamentally inaccessible.

The biggest problem, though, is that it's just not very well defined. In some contexts it's been defined by reaction, as you mentioned, though that definition has to be refined for more complicated applications (e.g. in anesthesiology where awareness might remain when reactions are suppressed.) Phenomenal experience and qualia are terms usually used to narrow the topic down to the mind-body problem, but even they have multiple definitions, and some of these definitions can lead to the conclusion that qualia don't even exist.

No problem :)

Maybe a bit, but atheists come in all flavors, and can even be religious or spiritual. Other times it's basically just shorthand for being a religious skeptic. I even heard a pantheist claim to be an atheist once, which tbh felt a bit over the top. It's a pretty flexible term, though. I think either pantheism or deism are the appropriate terms for what you're describing, if you want to call it a god. If you don't then I wouldn't overcomplicate it.

> All of the universe's consciousness condensed in one density prior to the big bang expansion.

Do you think its experience would be in any way analogous to what we experience? There's no reason to think this entity would have biological sensations, like our experience of hunger, so it's unclear what we might meaningfully derive from this claim even if we accept it as true. As you pointed out, it just results in a lot of "I don't know"s.

> does this density qualify as a god?

Gods are usually described as intelligent beings that interact with humans somehow. I don't think the sort of information processing required for intelligence is possible here. There's also no evidence that it has any direct relationship with humans.

You might be able to simplify by appealing to a sort of deism, but IMHO that usually just ends up making it less godlike. Of course, it depends on how exactly you go about it. Here's an argument for atheism that I made a while back using similar terms. What qualities do you think such an entity might have that could make it worthy of the title "god"?