Withers will show up no matter what you do if you don't recruit him at some point. It was the case even in EA.
Becoming Faithless has huge implications story wise btw, you know, that Wall of the Faithless that consumes souls overtime until there is nothing left? Yeah, it's still present. And the Dead 3 were returned into a lesser god status after the 2nd Sundering, not major gods like they used to be (which is why they cooked up this entire scam with the Absolute, to try to grab for more power and why Withers chastises them in the post credit scene).
As to other Bhaalspawn becoming Faithless? No, why would they, so long as they served Bhaal well. Bhaal is just really pissed off at good Durge for defying him.
Wasn’t that retconned or at least not mentioned? I know it was a thing in Neverwinter Nights 2, but modern books not even mention it, last I checked.
What about the likes of Imoen or other good Bhaalspawn? Sure, evil ones would not become faithless. However, what about those that weren’t actively evil or worshipped other gods?
Bhaal's plan was to get a lot of children, so that when he was killed, he could come back after his offspring slaughtered one another, while their essence returned to mana well, if I remember correctly.
In fact, you actually lose part Bhaal essence for a moment in BG 2 and you are just fine. BG 3 seems to contradict previous games, when it comes to narrative. Whole ACT 3 being unfinished aside.
