1. Re: Player experience. Yes, Gen Con has a place on the event submission form describing how experienced players should be. But GMs should be prepared to handle novices even if the event description asked for experienced players. Some people don’t look st the experience tags, while others have subjective interpretations of what constitutes “some” experience.
2. Every GM should familiarize themselves with the Event Host Policy. It answers a lot of questions GMs may have, including the ones you had about event start times. http://files.gencon.com/genconeventhostpolicy.pdf
3. I was surprised that there was no mention of playtesting your event in the section on prepping in the months before the convention. It helps a GM avoid potential difficulties: Are there any obvious plot holes? Are the PCs and their opposition properly balanced? Can they finish the event in the alloted time?
if you’re part of a gaming group at home, you’re not taking advantage of a great resource if you don’t ask them to playtest your game. If you’re not part of a gaming group, go to your local gaming store and ask if you can run the event there? Feedback and constructive criticism can salvage make a mediocre event good and a good event great.