Ol' Greatjon can kneel to participate in a challenge as a card ability. Can this be used if stealth was declared on him? Is he more of a multiplayer card, or a last second addition to a challenge?

Stealth prevents a character from participating in a challenge both through conventional (i.e. kneeling a character) and non-conventional (Greatjon's ability) means. However, you can trigger Greatjon's ability before Stealth can be declared against him.

Yes, he can be added to other player's challenges in a Melee. You choose which side of the challenge he commits too.

Thanks for the clarification! I'm definitely green at this game.