I find your edit an interesting one to think about and I largely agree with your edit.

Eventually it seems like you're left with skepticism of all things (epistemological nihilism).

But I don't mind that myself.

Do we know human reasoning is reliable?

Do we know reality is what it appears to be?

It's like I said in some of the other answers that I think nihilism seems true but only in a very abstract sense.

You talk a lot about definitions which I think is reasonable.

Like you said, how can you discuss ideas if you both mean something different?

If we use the word know to mean absolute certainty then we don't know anything.

I think most people use know to mean high degrees of confidence.

I'd say I know things with high degrees of confidence because of testable demonstrable repeatable evidence and various presuppositions like the Burden of proof.

I think existential nihilism and moral nihilism are based on the Burden of proof.

But it gets a bit trickier when you get more abstract and question the axioms and presuppositions themselves.