1. If I understand correctly, the Karzoug Statue being immune to the Attack trait means that when someone in my party encounters him, I cannot play something like Incendiary Cloud or Toxic Cloud. However, if I managed to use that card earlier on in the turn, could the person encountering the Statue add the dice from the spells to their combat check, since the dice themselves don't have the Attack trait?
Yes. The immunity rule prevents you from doing these three things:
Playing cards with the specified trait.
Using powers that would add that trait to the check.
Rolling dice with that trait during the encounter.
The cloud would have already been played, so you aren't breaking that rule. And you are using a power that would add the attack trait to the check, so that part is fine too. And the dice themselves don't have the Attack trait, just the Fire or Poison trait. So it work.
2a. I don't have the card text in front of me right now, but I believe the Roc has the power to be shuffled into a random open location if you fail to defeat it. If you fail to defeat a summoned Roc, is it shuffled into a random location deck?
No. Summoned cards always go back to the box when the encounter ends. See page 12 of RotR rulebook or page 14 of S& Rulebook, which say: "After evading or resolving all checks against a summoned card, banish it unless you’re instructed otherwise."
Instructed otherwise trips some people up, but it never happens in RotR. And when it does happen in S&S, it will be clear that you are being instructed otherwise. For example, if a card said "Summon and encounter a Roc. If you fail to defeat it, add it to an open location deck." That would be clear because the card telling you to summon it is the card instructing you otherwise.
2b. After an encounter with Iesha Foxglove, I've always returned her to the box after the encounter, because that seems like the natural thing to do. Assuming that is the correct thing to do, why isn't she treated like a regular villain and shuffled back into the location decks? I don't have the card right now, but I don't recall seeing any power instructing to banish her after the encounter, even though the Sandpoint Devil has text like that. In this sense, doesn't the rulebook have conflicting instructions regarding villains and summoned encounters?
Because summoned cards always go back to the box when you are done encountering them.