Well Together Retro and Together RPG are drop in drop out things that happen many times a year on a schedule, and are intended to be community play-throughs rather than a challenge, so it's up to the individual. In addition, Together RPG is currently... let's say on hiatus for the foreseeable future. That may change at some point, but don't worry about it for now. Shmup of the month operates just like TR, (though it doesn't seem to be active this year) and STGT is in November, which didn't happen here last year anyway.
So if this clashes with anything it's only Summer Game Challenge, and you can easily mitigate that if you wish by bumping it to August or September (though you lose the alliteration

). But it might not be a concern anyway. The two challenges are pretty different, there might not be a lot of overlap among players.
Um, anyway, all that babbling aside I think this sounds pretty fun. The only issue I see is that difficulty can be subjective, even among games that are supposed to be well known for their challenge. As an example, I might think that some super beginner level strategy game on easy mode is tough, while someone else might be good enough to whip something notoriously tough like X-Com Terror From the Deep. He'll correct me if I'm wrong, but if I recall Mr. Popo destroyed that game.
I know you kind of addressed this in the first point, but to go further my questions are:
- Do we need to set a standard?
- If so, how would it be done?
- What happens if someone brings up a game they want to play and someone else pops in to say it's too easy to count? Is that ignored or discussed?
Maybe I'm overthinking it though.