1) Yes security assist would be used to protect the files.
2) Yes, data assist would be used to manage the tables.
3, 4 ,5 6 ) When the user uploads the file, the file name should be stored in a database. by storeing the file in a database, you can later retrieve the file name. A files table would be a good idea. Using the users ID to relate the files to the user in the files table.
This way when the user logs in, you can crate a recordset to filter the files table on the users ID, and show a download link for those files that are associated to the user. You would use data assist to store the file name in the database.
See the "How to apply Digital File Pro to DataAssist Insert and Update pages " Tutorial on the DataAssist support page for more information: