I've recieved this same ban. I will admit I was signing in with a guest on Regicide to get the Ventator armor. I had never heard of the previous bans because I started in Reach and played clean. Halo 3 I didn't play multiplayer at all.
To Answer your questions:
1. The length of a ban depends. I've heard of people who get one-day bans and another who got a three-day ban.
2.The website URL is glitchy (FYI I'm kinda a computer nerd) so you CAN get to the website but you just have to try closing your web browser and trying again and again.
3. As Blacklight said 343 are kinda closed off. Meaning they have no phone number no email and no...well ANYTHING to contact them with.
I bet 343 are laughing this ASSES off with all the ban talk. They probably aren't even trying to fix it for unfair bans (Yes I belive soem people out there are unfairly banned).
Thanks for your time,
Overated Fishdy