Are you able to grab a dark pact during your upkeep battling the ancient one? Also are you able to exhaust/discard dark pacts during battle with the ancient one to gain power tokens? Not sure if I'm using the Dark Pact cards correctly while I'm battling the ancient one.

I think it was said you cannot gain power during the Final Battle (even if you can still spend the power you acquired during pre-final battle game), but for an official word on this point we should wait the coming FAQ. This said, since you cannot gain power during Final Battle, it does not have much sense taking a pact.

In the current version of Lurker, you can take dark pacts whenever you want, including during the Final Battle and even still gain power. However, this has lead many players to feel that the Lurker acts more like a guardian than a herald. So it has been said by FFG that the upcoming FAQ will state that no power can be gained during the Final Battle. And dark pacts can never be discarded, ever.

Something else to note was that the expansion with the first-ever herald, The King in Yellow, said that the herald was supposed to be discarded when the AO awakens, since "it will not have any further effect." However, if you're playing against Glaaki, an AO that was released before the KiY expansion, the King in Yellow herald absolutely would have an effect during final combat.

No other expansions that have been released have mentioned discarding a herald when the AO awakens.

So the question arises: are heralds supposed to be discarded during final combat, and each expansion's printing was an oversight?

Or are heralds supposed to stay in play the whole game, and the "no further effect" phrase simply didn't account for Glaaki?

In my personal opinion, a "herald" is meant to awaken the AO, and has no further effect once the AO is in play, so they should be discarded (of course, herald effects related to final combat are still regarded and cards inflicted by the herald still remain in play).

However the purpose of the Lurker is to awaken the AO while also striking deals with the investigators. Once the AO awakens, I believe all deals are off. Bound Allies get attached to the AO and the Blood and Soul pacts get discarded without use. Alternately, investigators get to keep their Blood and Soul pacts, but may no longer gain power tokens during combat (this part is essential because otherwise you could produce a no-lose combat situation, which is hands-down the worst loophole in all of Arkham Horror).

Yes, it was quite an oversight. However, I would prefer to not have the herald discarded. As you mentioned, it could make for some pretty gnarly combos like Glaaki + KiY. With Lurker, I would like to see an implementation of a mechanic where reckoning cards still get drawn during the final battle. After all, since the pacts aren't discarded, the Lurker should still be trying to claim his debts, but also, no power can be gained by using the pacts (only through the reckoning cards).