Champion events & crash
1) No. If you crash before the event ends then you will not get event credit and reward.
2) Yes. Being in a party helps. Or if you recall people in chat and join up on them. If you’re in a party with someone on the original map shard – just right click on their party window and join their map. Taxi service can help if you don’t land back in automatically.
3) There are unique items that drop for many of the World Bosses…. but I believe most can be purchased from the TP. However some of the collection items in particular are so expensive that a new player would most likely not be able to purchase them anytime soon. Other “champs” like Tequatl and Vinewrath have items that you can only get from them and are account-bound.
If you haven’t already – turn your graphic settings down. Helps most people.