Hi James.
1. You don't need a specialised FileDocument linked to each user. If you don't commit the file then you shouldn't need to delete it once upload is done. The MF is simple, with two actions Create Object FileDocument , then Show Page.
2. The DataView will already have your Newly created FileDocument as a context object, since you supplied it as a page parameter, so you don't need to fetch it again.
Add your 'SubmitToAPI' button to the same DataView and the file will be available as a parameter to your MF
3. You might (probably will) need to use the REST services module from the Appstore to add your file to the POST request (See RESTServices.post).