I've read where Osama denied having any involvement.
I've read some lengthy interviews with Osama back in the day. His views and objections were quite different from what the western media portrayed them as. Intelligent, articulate, but yes, a hard liner. As I recall, (and this might be faulty) his greatest upset was America "occupation" so to speak, in Mecca.
You can ask for proof of a "conspiracy" (ie. an alternative conspiracy to the generally accepted one), but there is scant proof of the generally accepted conspiracy as well.
We can discuss things. But demands for "proof" from either side will have a predictable course. Predictable as the existence of a deity.