1. Good question, so: create a dummy student account, submit an assignment, delete the account, see if you can still see the assignment submission in the grade book. Better still if done on test server then can see if moodledata folder increases then decreases.
2. Not that I know of, would be cool to be graphical.
3. Reduce the number of automated course backups, reduce file upload sizes, if '1' removes space then prune old accounts (including teachers and managers), look at course usage stats for inactive courses and identify a list to check with the creator - if then stale then backup and delete, if server on Windows then can use file compression (perhaps on Linux too), don't allow uploads of uncompressed formats like avi - insist on mp4 etc. - filter the mime type etc.
4. If the Moodle Association kicks off, fund through that.