You can set up a product to sell anything, even a download at a later time.
But if you are offering the download at a later time you won't be able to take advantage of the download links that we can normally generate at the time of purchase. That would leave you to take care of delivering the files to your buyers by other means, perhaps by giving them a link to your own site or by using an outside file hosting service, and you'd have to provide your own way to verify that the downloads are only going to people who have already paid.
As for paying a commission to the experts that will be participating in your teleseminar, all you would need to do is set them up to participate in your affiliate program. Set up affiliate accounts for them, or have them set up their own, and then you can provide them with links that will track the sales they refer to you before and after the event.