If completed against Hastur, and then if Hastur wakes up with the terror level at zero.. is Hastur then a +2 ? (Or similarly, Yibb-Tstll waking up at zero clue tokens on the board.)

We have no ruling on this question, therefore my answer is unofficial, but it seems fair and logical to go the way you say. Missions are costly things to pull off, mostly in terms of time, so I would allow the completion to give the team a break. Not that they should need it in either case. You could take the view that a reduction of nothing is nothing, but that seems exclusive and sets a precedent not actually established elsewhere in the games card pool.

I don't see why not. Nyarlathotep's modifier is –4 , and Sealing the Beast's Power means it's –4 + 2 = –2 .

Likewise, Hastur's modifier is –X , so Sealing the Beast's Power means it's –X + 2 = –X+2 . For a terror level of 5, for example, it becomes –5+2 = –3 .

Agreed. Additionally, if you consider some monsters (Cultist, for example) they have a positive combat modifier, giving you basically extra dice for free. So, no reasons why an AO could not have a positive combat modifier as well

So, basically, if you have StBP and Awaken the Ancient One--you could conceivably end the game as quickly as the number of rounds it takes you to finish StBP

Yes that is possible, and its one reason why many people dislike Awaken the Ancient One.

Well, it's a hit or miss in my mind. There are just far too many cards in the game, especially if you have every expansion, to make me dislike a card for being broken since the odds are just too great against drawing it. On top of that, it's a Unique Item, and I don't think there's anything that let's one just pick through the deck for whatever Unique Item you want outside of getting lucky in an Encounter so--meh?

It is a rare-ish combo yes. There are actually a few ways to search the decks for specific spells or items. One sure way is to start with Daisy Walker (librarian from KH) who has a tome that lets her find any spell. The mission is harder to fish for though.

Even so, I dislike the idea of AtAO thematically, it seems too exploitative.

The problem is more with base game Hastur and his -X modifier. In the base game, it was easy enough to keep the terror level low and coax him to awaken.

If you want to solve your SAO/Hastur problem, I suggest starting with the updated Hastur .