0 = -x + x
This means two things, and it is actually not quite “something comes out of nothing”. It really means superposition of everything is equal to nothing, not identical and not because it is actually nothing, but because it is effectively nothing.
Totally chaotic arrangement of everything cancels out itself, it is there but without an effect or defined form in any direction or dimension. It means nothing actually implies everything, it can not exist without it like there is no black without white.
No yang without yin. So nothing is not simply nothing, it is also everything. And another duality is that what comes out of that “nothing” can not be just any old something, it must be paired into two opposite some-things. It’s undeniable! — Zelebg
First, duality. Why is it that the concept of duality has emerged in both Eastern and Western thought? Daoism has yin-yang, Buddhism has the middle-path and Heraclitus too spoke of opposites and don't forget Aristotle's golden mean. If this means anything then it must be that the dualistic model is more than just a culturally idiosyncratic point of view and that it's a good representation of reality.
We can only guess the origins of dualistic thinking but one thing is clear: duality is about extremes. Look at dualistic pairs e.g. hot-cold, large-small, create-destroy, etc. Each pair expresses extremes of a particular property a thing may possess: hot-cold are extremes of temperature, large-small are extremes of size, create-destroy are extremes of existence, etc. Given this is so, what do we make of the in-between stuff e.g. between hot and cold is warm, between large and small is medium, between create and destroy is preserve, etc.
The existence of words that capture the stuff between dualistic pairs betrays the fact that dualistic thinking doesn't quite give us the correct or, if you prefer, the complete picture of reality. Nevertheless, dualistic pairs do provide a useful guide to where we should place ourselves in the universe - right in the middle where things are comfortable and exciting because all phenomena seems to occur between lower and upper extremis points. Life, as we know it, must avoid extremes to survive. In a sense then, all the dualistic philosophers I mentioned were deliberately or unwittingly affirming the Goldilocks zone for life. This also means, quite oddly, that dualistic thinking is an implicit claim of not a duality but a trinity, the positive, the neutral and the negative; it is best conveyed by the Hindu trinity of Brahma the creator, Vishnu the preserver and Shiva the destroyer.
One more thing I want to mention is the concept of negation; it has the ability to split the universe into two, a thing that is affirmed and the rest that is negated. Is this duality? Yes, because an entity and its negation are definitely opposites and no, because these opposites aren't always extremes. The negation of cold is not-cold and that, despite referring to the dualistic opposite hot, also refers to warm. This isn't useful since it fails to point to the other extreme. Imagine if the words "hot" didn't exist and someone were to claim that a cup of coffee is not-cold. Is that enough information for you to take a sip of the coffee? No, the coffee could be hot and could burn your mouth. Negation, though technically dualistic in nature, isn't adequate; we need a separate dualistic worldview based on explicit consideration of extremes.
Secondly, the matter of nothing, something and everything. You made an amazing argument and I really liked the -x + x = 0; expressing duality mathematically makes the whole issue clearer. I'd like you to imagine a world made of 6 things: a, -a, b, -b, and c, -c. Everything would be {a, -a, b, -b, c, -c}, and something would be a combination of these but not all of them. Nothing would be again {a, -a, b, -b, c, -c} because every object and its opposite would cancel each other. So, everything is nothing.
Duality would impose the condition that everything have an opposite and that includes everything itself and also nothing. What is the opposite of everything and also the opposite of nothing? If one is to be dualistic then we have to talk in terms of extremes; if we take everything as one extremis what would be the other extremis? Wouldn't it be nothing? Your mathematical treatment of the matter produces a beautiful answer to the question.
Everything (e) = nothing (0)
Opposite of everything = -e
1. e = 0
2. -e = -0 = 0 = e
3. -e + e = 0
4. e + e = 0 + 0 = 0 = e
The opposite of everything is nothing and that too because everything is nothing.
My concern is whether a mathematical representation of the issue is apt or not. Can I speak of duality in terms of positive and negative numbers?
Well, nothing, something and everything appear quantitative and we can express it mathematically as follows: nothing (0) < something (at least 1 but not all) < everything (the entire universe). This is an accurate mathematical translation as far as I can see but it lacks the positive-negative feature of numbers your argument has. Notice that in my mathematical model, dualistic thinking assigns the opposite of everything as nothing since they're extremis points. However it doesn't allow me to conclude that nothing is everything as your model with positive and negative numbers does.
Anyway, if there's any problem with the -x + x = 0 model of reality, it's the reliance on negation to "take care of" the stuff in between -x and +x since, generally speaking, there are no opposites for the in-between: for instance hot and cold are opposites but there is no such thing is an opposite for warm unless you say not-warm but that creates 2 possibilities, hot or cold, raising the count of states that can be obtained to 3 which is not a duality but a trinity. Perhaps it's a trivial truth.
What else?