Suppose there only ever existed one indecomposable, irreducible object.
What could distinguish it from nothingness? From not existing, as there is nothing besides it that could deduce its information?
Itself?
But then it must somehow fetch its own information, but since it's the only thing without any change existing, it stays identical and no information is fetched.
You are conflating the idea of existence with the idea of knowing that something exists. Suppose there was a Universe which contained nothing but three irreducible blobs and a person who is able to see the blobs. If the person dies, do the three blobs cease to exist?
Imagine empty spacetime and in addition either a single electron or a single photon. The elementary particle has properties which empty spacetime does not have: Energy, momentum, the electron has in addition a spin.
Why asking further questions about information? These physical properties do not presuppose an observer.
People have proposed universes consisting of a single particle that goes between the beginning and end of the universe and back to the beginning again, only this time taking a different path so it appears to be another particle. That does not seem to correspond with our universe, which has a beginning but seems likely to expand forever.
Otherwise, you would seem to have the Creature from the Abyss of No Dimensions from Flatland. It knows nothing outside itself, but it is complex enough (somehow) to have an intelligence.
A less extreme example might be an intelligence that exists within another system with no external communication. The world telephone network is about as complex as the human brain, with a similar number of nodes, and a semi-random structure. It could be routing data for us, but also thinking. But what would it think about?
The difference is made by itself and nothingness. Difference is just lacking a property which one has, or having a property which one lacks. The distinction is quite simple: nothingness has no property, that thing has one property (itself), since nothingness has no property then it lacks that property, voilá.
What could distinguish it from nothingness? From not existing, as there is nothing besides it that could deduce its information?
What you may have said there is that, if there is one thing, it is nondual. Few people think non-duality is the same as not existing. Aside from that, it reads like you are trying to argue for a conclusion you cannot think, let alone believe. If so, there is a small amount of self referential felicity to that (being unable to think of being unable to think of something), but you're not about to convince me that only you exist, so hey hum.
The relation of pure being to pure nothing, is that they are both negations of each other.
In a more Hegelian sense, they are both implying and negating each other. A pure nothing can only be a nothing insofar there is something else in relation to it, a something which is not nothing.
These are their only attributes, it has no capacity to deduce any information, nor is there any information to deduce.
