documentary - Outside of the movie, what evidence confirms scoring battles of "King of Kong" are real?

Very intriguing question. I haven't seen the film--only discovered its existence a few days ago--so while I won't understand all the context I may be a little more impartial. I have a few comments and have also done a little research.

Firstly, regarding the "C" of young Billy Mitchell's top. It just seems to me that the left half is folded underneath a crease, like the "2" of "25,000,000" directly below it.

Twin Galaxies seems to be by far the main compilation of gaming records, statistics, etc. Its Wikipedia page contains a lot of its history dating back to 1981, however this is not entirely convincing for the same reason--a lot of its references are from TwinGalaxies.com. It claims to have collaborated with Guinness World Records; the link is dead but an old version can be seen here which confirms a lot of the history. Additionally, performing a whois on TwinGalaxies.com suggests that the site has been up for at least 9 years (King of Kong was released in 2007). So to me it seems like a legitimate entity.

It's hard to tell with Mr. Awesome because I can't believe anyone like that could take themselves completely seriously. From what I've researched, my opinion is that Mr. Awesome is Roy Shildt's half-serious persona--and the serious half reflects Shildt's willingness to do anything to be famous. I found an appearance of his on the Howard Stern Show here (it's Stern, so it's NSFW) in 2001 (source here, and do a search on "awesome"). Again it seems to not be created specifically for the film.

Seth Gordon's biography suggests mockumentary. As well as what you've mentioned, he apparently cites as an influence, Christopher Guest, who is well known for his work with mockumentaries such as This is Spinal Tap. I can't come up with any examples of documentary directors who have later worked in comedy, although I don't think it's impossible.

All of this evidence considered, I think King of Kong is genuine. If you believe Guinness WR and whois, then you should recognise Twin Galaxies. And if you recognise them you should believe the information they present. I guess it could be a hoax, but it would have to be incredibly well thought out years in advance, as well as executed perfectly.

Now I should really go watch the movie =)