I revealed Chieftain of the Pit during setup of The Seventh Level. His text says "When Revealed: Chieftain of the Pit gets +3 [attack] until the end of the round." I'm assuming that I just ignore that since round one hasn't officially started yet. Is this right?

That's how I would interpret it. The round starts with the resource phase so I would say he doesn't get the +3 attack bonus

i would agree with that interpretation

There are a lot of nasty 'When Revealed' effects that can be avoided during the Set Up. It is nice to see them get drawn at this time. Of course, others can still have an effect. The Set Up is like a mini round, and Round 1 starts after Set Up has all been applied. You can use cards like 0 cost Events or Eleanor if appropriate, but Set Up is the only phase, so Eleanor wouldn't get Readied. Cheers!

I'm not so sure you can play cards in setup unless they are a response effect and cost 0. The rules are quite clear you can play responses (like Eleanor's effect) but cannot take actions during setup.

FAQ 1.19

"When Revealed" effects are resolved if the cards are revealed during setup. A player can trigger responses during setup, following the normal game rules. Players cannot take Actions during setup.

That's correct- you could use a 0-cost Response event, but not a 0-cost Action.

Sorry, I may have misled people. I should have specified '0 cost Responses' like Eleanor who will not ready, but it would be useful against something like 'Old Wives Tale'. That will exhaust all Heroes! Could be a disasterous start! Cheers!