My read was that the MC cares more about Erevi finding out than Nergal, it's just that Nergal finding out has downstream ramifications for her since she is one of his faithful. In that line of thinking, nothing about the MCs motivation has changed. He just finally met someone willing to blackmail him. Erevi still has no idea that he's been involved with the other temple, and I'm pretty sure Namaah openly suggests spilling the beans. That secret and the fact that the MC+Erevi's daughter brainwashed her into having more children are both bridges left to be crossed. Namaah's threat puts both his relationship with Erevi and her safety at risk.Potentially controversial opinion:
Yes, he's horny, young, and sometimes stupid, but he cares about every one of his girls, and can be smart and unbending when it comes to defending them.
Naamah too, seems like she would be smarter than kidnapping a mother of MC's children, and expecting cooperation of any kind instead of seething resentment, and it's only by the power of plot convenience that she gets away with it, instead of getting his attention in some other way, and then playing on the same facts she laid out anyway, that she holds some leverage over him in the form of information she could feed to Nergal. And, even though this is a lesser issue, why does she even exist when we already have Erevi's daughter? Story reasons aside ofc.
Alice has always been a masochist, so I imagine the idea that her captivity was more of a playful romp was meant to soften the blow. Either way, I don't think the direction Tinkerer took was out of character for anyone involved. Had Alice been injured I'm sure the "relationship" with Namaah that takes place after the quest would not be the same.
All that aside, as powerful as the MC has become he still isn't a match for Namaah on paper. The idea that we finally met an entity capable of wiping the floor with him (for now) is a needed reality check if the gods themselves are coming into play soon.