I'm a little unclear as to what the disagreement is.

I still think in your scenario, we would not be able to prove that that is what actually happened as opposed to someone manipulating the inputs to the brain to make it think that's what happened. Or to change it completely.

Like that's the entire solipsistic problem. The brain is just sort of self-contained. Everything is essentially input. We are incapable of really knowing the experience of someone or something else. Or if what we perceive is what's actually happening. Because, as you note, the brain can be manipulated to act in ways counter to the manner it had been acting previously. We can induce sound and vision. If we were better at it and could keep the brain alive, we could full on do a brain in a vat.

And I agree that sort of proof is impossible to acquire. Like, there is no way to prove conclusively that anyone else is conscious. But that's why we instead treat it as an axiom instead. We basically say "We're not going to prove it, we're just going to accept it as true". And that's fine.

Mostly because it not being true is an existential horror no one wants to truly confront.

And even with all of the more absurd conjectures, basically any form of "Last Thursdayism" of which simulation theory falls under, it's quicker to just assume that it's not true and treat the world we perceive as real with real rules that can be discovered through observation, testing, and experimentation. Because that's the only way we've been able to make any progress.