As I am not a Riot employee I cannot say what is or is not in the Void. I am merely offering a possible explanation for the Void. Also I would like to thank you for the stimulating discussion. :)
At the Mountains of Madness is not about other realities but it does mention the idea. The big C (Cthulhu) and his spawn come from a place so far away as to be alien to the Elder things. The general consensus is that things of that nature aren't wholly from our reality.
On the first point... it's a rule that something needs to come from a place or another thing. Nothing doesn't have those. Therefore nothing could just spit out matter. Of course at that point we have something so it isn't likely to happen again. But it could happen.
My response to the second point would be that the Voidborn only have something comprehensible in our reality. The point about nothing was not that the Void has nothing, just that the Void may be lacking some of the things we have in our reality. And currently, yeah it seems Riot is going the way of "the Void has matter, and space, and time, and other stuff" but they do not have to. In theory they can still do the opposite. They haven't been very clear on it, probably because they haven't got it nailed down yet. They may, (and in fact probably) will go your way in the future. But that does not mean it is the only viable way for the Void to be. To be clear, if the Void didn't have space or boundaries then the Voidborn would not need these things while in the Void.
Descriptions like that kind of back my point up. They don't say anything meaningful or deeply descriptive. It's merely there to show you that the character viewing the thing literally cannot describe it effectively. They have no words to capture what they've witnessed so they default to words that apply a label to a concept that cannot be experienced. The point is not to be dramatic but to drive home how bizarre and horrific the things being witnessed are, at least in Lovecraft's case.
But in the Mountains of Madness we don't learn much about the Elder Things and we don't see the inner portions of the city. Just the tiniest bit about their history and a bit of the outer sections. I personally have nothing against writing about the Void, in fact I love the idea, and love doing it myself. However, it's a thin line too walk. Explaining too much turns the Void into "scary purple monster world" which I feel does a disservice to the inspiration. Basically I want them to write about it, but I'd prefer to keep it limited in scope. I'd love to hear more about Icathia and what the Voidborn are doing on Runeterra.
I would like to make it clear, that I am merely offering an alternative take on the Void and that this doesn't mean it can't be discussed in universe. Just that describing the Void itself in terms that make it too similar to Runeterra, and in fact our reality takes away from what the Void has already claimed to be.