Yes, you can use the redirect url field to have the form be a requirement before a file download (just put the file download on the 'success' page).
Note that there's no way at present to protect the success page from direct access. A cookie or session variable would have to be set upon a successful form submit.
I'm not sure why you'd want them to fill out the form (auto-populated or not) and create a duplicate lead? If they're logged in, you could just set a user meta field to 'registered' or some such when they fill out the first form and allow them to bypass the form.
If you want to update their lead/contact when they request subsequent files, that'd be outside the scope of this plugin (it can only create leads and does not track which lead belongs to which user). Sounds like an excellent idea for an add-on though!