The poll was conducted by a bunch of major broadcasting organisations including the BBC. Given the beeb's editorial bias on this issue it is a bit much to suggest it is distorted to be pro bush/blair.
In terms of the quantities he could have manufactured, the answer is enough to kill a hell of a lot of people.
Re: Iraq
Indeed. It is merely a possibility. It is however a plausible one. He may or may not have had large or small quantities. Equally, the fact that he did not use them, even at the end could mean many things, of which the possibility that he didn't have any is but one option.
The truth of the matter is that we may well never know. Finding nothing unfortunately does not prove matters either way.