To answer your first question; "Is there a particular page on our site that many people look at that causes them to request a demo?" I have used tools outside of Marketo. The tool that I used was Google Analytics. This provided the path's people were taking before the form completion. I am not sure if you can get this data from Marketo without analyzing each lead that interacted with the form and downloading each leads activity.
For the second and third question, my approach will be to setup a smart campaign for each touch (email, ebook download, video view) that looks for request demo form completion and set the program status to success. This provide the assets that the lead interact before fillout the demo request form.