Let’s say you had to cancel your event because of the weather. Here’s the answer to two of your main questions:
Yes, everyone will get a notification that it was cancelled. They will receive a push notification in their app if they have push notifications turned on, or and e-mail if they don’t have the app, and signed up with their email address.
No, players will not automatically get credited. You have to go through and manually credit everyone through the “Manage Users” section.