Disney is not blocking the release-- they've given more than a year's notice that Miramax should seek an alternate distributor. So why didn't they?
And I know, of course, why Moore won't pony up the money-- since he'd lose it all. More than likely, Disney knows that documentaries generally don't earn as much as they would cost to promote and distribute (although they've now taken care of the promotion part). Why not just wait and release it on DVD? Probably much more profitable in the long term.