Nick repeatedly gets his ass handed to him over the course of the movie - having his head rammed into multiple hard surfaces, being thrown about like a ragdoll into stone walls, being ejected from a rolling ambulance, getting punched so hard he becomes vertically airborne and hits the ground a good ten metres away - any one of those things should have put him in the hospital, but he manages to remain mostly unscathed. Not only did the curse let him survive the plane crash, but it's also allowing him to take ridiculously large amounts of whoop-ass, in order to keep him alive to be stabbed with Set's dagger.
At the end, just as Nick is about to smash the stone again, Ahmanet begs him to stop, saying that 'all will be lost' and that he will be 'a living god'. Why would all be lost to Ahmanet if Nick destroys the stone and becomes a living god? Isn't that what she's been wanting the whole movie? Additionally, she doesn't believe all will be lost to Nick either, because she says that he will thank her when the ritual is complete. So, why wouldn't she stay silent as he destroys the stone? The answer then is that Jenny was right, Ahmanet was bluffing, and destroying the stone would have undone Set and broken the curse, but would have left Jenny dead. However, not only is the altruistic part of Nick now invested in the possibility of bringing Jenny back from the dead, Ahmanet is still exerting a level of control over him as well, hence why he ends up stabbing himself and completing the ritual.
This is completely wrong - Ahmanet is trying to entice Nick by saying that if he doesn't break the stone and instead joins her, then he will be a living god and will thank her for it. Unfortunately for her, this makes him twig to the fact that if he becomes a living god with the power of life over death... he will be able to bring Jenny back from the dead. Oops.
Well, her exact words before Nick tries to smash the stone again were "Wait. Destroy the stone, and all is lost. You will be a living god. You will have power of life over death." That sounds like smashing the stone gets him stuck hosting Set, just like completing the ritual would.
There's an implicit "on the other hand" in between "Destroy the stone and all is lost" and "You will be a living god."
Note that the pier where the Mummy crash-lands and the ruined abbey that Nick and Jenny end up at are two different places, adjacent to each other, since the plane crashed across a large distance. This is why Jenny keeps saying that the abbey is not the primary crash site and that the artefact (the sarcophagus) isn't here. Nick's come to the abbey because Ahmanet's in his head and he's being drawn to her corpse and not the sarcophagus: the Mummy has crawled across to the abbey from the ruined pier to find the dagger of Set.
Ahmanet was conscious and living in the most complete darkness, with her power weakened by mercury and alone. Even if she's corrupted, it's not so strange to suppose that she isn't mentally functional after her awakening. She probably feel how her body decomposed and dry for millenia, as we know in Prodigium, she can feel pain.
At least that Set was talking to her in her imprisonment.
