First off, I'm going to point out that the “Defeat a God” challenge (as well as the other challenges released throughout the block) are pretty much casual variants. In fact, here's a quote from the Hero's Path FAQ (found here:
http://www.wizards.com/Magic/magazine/Article.aspx?x=mtg/daily/feature/HerosPathFAQ#defeat):Q: I have a question about a weird interaction between my deck and the Challenge Deck. How do I resolve it?
First and foremost, the Challenge Deck experiences are about having a fun and unique experience. If you have an intuitive sense of how to resolve that interaction that feels fun, do that!
As for your specific questions:
1. As you can see on the reminder text of the Throng, it says that ability will not trigger when the game begins. So no, you will not get the trigger for the Throngs that start the game on the battlefield.
2. Yes, you reveal them both, then the player making the choice for Xenogos's team will make the decision in which order the spells are being cast in.
3. Remember, this is a casual format. You probably want Xenogos's team playing to win. While not assigning the extra damage is optional and up to the player controlling Xenogos's team to make those choices, you probably want him to be playing to win the game, so you probably want him to trample over if able.
4. Yes, it creates a delayed trigger that goes on the stack at the beginning of combat step. It will affect Revelers that enter after the Charge has resolved.
Nathan Long
Wizards.Com Boards NetRep