Second,would be that only one download per IP address should be counted. Yes, people might not get the file the first time due to errors, but you've still been paid, you shouldn't get paid for their reattempts.

Bad idea (there are a lot of people using proxies - means a lot of people having the same IP at the same time). Better is using sessions and bulding a user history. It isn't required to offer downloads to non-registered users if a fast registration system is in use.

Third, would be to continue hand inspecting the logs before payout. This should prevent people from using an internet cafe etc to generate huge download counts since a large amount of IP's from the same area would throw up a large warning flag.

Ha ha ha! :) There are so many ways to implement "click-bots". Using an internet cafe is what the clueless are doing :)

To have a picture: Create a webpage (simple HTML) with something a lot of people are interested in ([censored]? Games? Music?). Advertise this webpage. The webpage contains an invisible frame requesting a page from ShareCG. Everybody visiting your webpage generates a "view" on ShareCG with his own, unique IP-Address. If a download is addressed, this download is started (and mostly interrupted by the surprised, innocent user).

Another: A lot of hobby-forums allows using images and/or iframes in comments. Anybody who accesses a page with a prepared comment generates a view on ShareCG.

Ergo: No way around offering downloads only to registered users.