We know that the British increased poppy cultivation and manufactured a monopoly on opium for the purpose of export to China. Without it, I don't see how the Chinese demand would grow at the level it did. I have no idea what "proof" would be, an alternate world without British interference?
>How could the British have manufactured demand, if they could not even enter China outside of Hong Kong?
Guessing you meant Canton here, by smuggling/buying off locals.
Having a monopoly on opium doesn't make people want more of it. Smuggling doesn't make more of it, either. What you're describing are both forms of taking control of supply, and don't speak to demand.
When one supplier takes control of supply, that doesn't generally lead to lower prices; but expanding production -- which the British did -- certainly can.
That China had some opium use without British influence doesn't mean the British did not manufacture a demand for it.
We can't take it as given, without proof that the British did manufacture a demand for it. This is a form of "argument from ignorance". Lowering prices is not manufacturing demand. How could the British have manufactured demand, if they could not even enter China outside of Hong Kong?