Cards have the same artist if there's an artist in common among them. For example, you could exile four cards illustrated by Zoltan Boros and one card illustrated by Zoltan Boros and Gabor Szikszai. However, you could not exile two cards illustrated by Zoltan Boros, two by Gabor Szikszai, and one by both of them, as those five cards don't have a single artist in common across all of them.

You choose whether to cast the chosen card as part of the chapter ability's resolution. If you choose not to cast it at that time, or if you can't cast it (perhaps because it requires a target and there are no legal targets available), you won't be able to cast it later. But maybe a later chapter will randomly choose the card again! Hope springs eternal under the big top.

In Un- games, you can choose any applicable cards from outside the game, even if they're not part of your Draft or Sealed pool or part of your Constructed sideboard.