First off, I definitely recommend you review the HOST page, if you have not done so:
It has information that might help you. Specifically, you'll be using the Event Messaging Tool, which will appear on the Event Details page shortly, to send that information to attendees. It will look like a forum thread, with your message to registered players at top. That's what you'll use to communicate the actual links to your players.
There will be at least one LFG channel on the Gen Con Discord.
You probably should wait 10-15 minutes post game start to call your game.
There will be some help channels on Discord; what the issue is would determine whether you'd be using the Discord or emailing us. For instance, in your example above, you'd want to email us letting us know you were ready to run the event, but not enough players showed; that will prevent it from being marked as a GM no show. If you're having trouble with a specific platform, there will be some channels on Discord which would be helpful for that. I do not know at this point what sort of Customer Service presence will be on Discord, I can only speak to events. We will monitor what we can, but for anything that it's probably important we have a record of, you can email us.
Definitely, we'll have more info on the Discord in the future, and it will open before the show.
Gen Con LLC