Without going into too many details, living in urban areas does not necessarily make one urbane and nor does having plenty of wealth.
We see ample real life examples across the globe of rich people and families following superstitious beliefs. And be it the Ambanis or the Queen of England, what we know of them is only what they want us to see of them, nothing beyond. That doesn't mean they're the most progressive of people just due to their wealth. There are plenty of leaked stories that imply otherwise.
In Narmada's case, if there was and is one thing consistent about her character, it is wanting her children's happiness – so she was okay with accepting everything that came their way, so long as it meant her children were happy.
For the longest time, she wanted Aryan to get married and settle down. But that man did not even pay attention to a woman enough to consider her as anything. So the day he brought Imlie home, Narmada did not care about anything with regards to her background — all that mattered was that she was a young girl who her son had shown enough interest in to bring her home and that was hope enough for her.
So she just chose to see the goodness in her and the happiness that she brought them with her presence and shenanigans. Even when Narmada learnt of Imlie going through a divorce, she was guilty about her actions but she never gave up hope when it came to Imlie and Aryan.
After Arylie's marriage, the reason she never cared about Imlie’s shenanigans is because Aryan was always with her and he was happy.
After the shooting incident, she still had not flipped until BM brought her face to face with a fear that was stronger than her son's happiness — that of losing her son and Imlie being the reason for it. That's when her worst fears came to life and she stopped caring about her son's happiness and just cared about her son being alive. Could the bold have been shown better by the CVs? Yes. But the CVs rushed through it all and relied on the audience making that connection because we've seen her pain at losing her husband (when narrating about Arvind's death to AKT), we've seen her pain at Arvind's death and the impact it had on her kids, and we've also seen her fears at the thoughts of her losing Aryan, or even Arpita. All combined together, it is not difficult to understand why Narmada flipped. The problem is, because the audience had seen her all sweet and accepting, we couldn't accept her flipping, but it's not that unrelateable.
Additionally, I'm not sure about the very recent episodes post confession as I haven't watched them yet, but whatever I had seen of Narmada until Aryan returning home, she wasn't firm on the fact that Imlie is the problem, she was even guilty of her actions, in fact – she always needed to be pushed in that direction by BM before she acted on it each time.
If they stay on that approach, it won't be difficult to whitewash Narmada because the way they're portraying her character is that she is not bad/wrong, just someone who's weak enough to be easily manipulated when her children are concerned.
Again, with the announcement of the arrival of a grandchild, the focus shifted again to the 'happiness' of her son, but now attached more to having a child than a wife of choice. And again, she's okay to do anything or go any lengths to secure that happiness for her son/family and hence, the dancing to BM's tunes and spewing the BM-fed filth. And once a person is down that road pf blindly following another, it is quite difficult to come back. We will continue to see this flipped Narmada, at least till BM is around.