I don't know the complete backstory on this issue (ie have only read this thread), but I've seen Wikia (
http://www.wikia.com/fandom) used in these kind of situations before. While targeted more at fictional universes etc I think it solves many of the aforementioned problems. Wikia seem to have the spam fairly under control, and are unlikely to disappear any time soon. I think the main downside is having to deal with ads, but imo a 0 cost solution that ticks all the other boxes isn't a bad start.
In fact, there are already two FIRST Wiki things on their (
http://firstrc.wikia.com/wiki/FIRST_Robotics_Wiki and
http://firstrobotics.wikia.com/wiki/Main_Page) would it be worth chasing up what's happening with one of these, or starting another one?
Even if Wikia seems like a bad fit, there are more than a few (
https://www.mediawiki.org/wiki/Hosting_services) Mediawiki hosting services, both paid and free, that will take the brunt of the work out of the equation.
I think this is a bit of chicken before egg problem. It's hard to get people contributing to a wiki when there isn't any content to begin with, and it's hard to get content when there isn't anyone contributing. The challenge is getting contributors/content to a critical mass where it's a useful resource that's valuable enough to maintain, regardless of the platform it's built on.
@virtuald, I'd be happy to work with you or anyone else on looking at any of these options.