I might be among the few who will not be surprised if the precap is not a dream and Narmada did slap Imlie as an impulsive reaction. Not trying to justify her and I am not at all happy with the slap part. Because, an abuse is an abuse - be it physical or emotional and sometimes emotional abuse has a long lasting impact on a person than physical abuse.
Now coming back to Narmada, I have been expecting such a situation (not a slap, but some cold reaction) since long. Her extra sweetness in the previous 2 occasions when Imlie had put Aryan & the Rathores through public humiliation, was really indigestible to me - first on marriage day when she left the mandap and second on reception day with her Jhadu antics. It made me feel like Narmada was unnecessarily made into a mahan then. Because, that's is not how a parent behave at such insults in real life. Only we knew about the inner dynamics of Arylie then and why Imlie behaved so; but Narmada didn't. So, when in both these occasions, Narmada was shown to be more in support of Imlie and that CVs didn't use BM to instigate her then, I was like daal mein kuch kaala hain. Since then, I was expecting Narmada to turn against Imlie any time, especially with BM still around. And this time, she has a valid reason to go against Imlie. She had warned Imlie not to step out of the house especially since she was just out of danger after the fire incident. So, to her it is Imlie's zidd that has costed her her son's life this time. Hence, It's quite natural that, any mother who has her share of fears & insecurities after loosing two of her family members, will fall prey to BM's/Jyo's manipulative deeds. Especially when Imlie didn't disclose to her that she was going out to search for donors. CVs could have avoided the slap; instead they could have shown Narmada stopping Imlie from meeting Aryan or something of that sort. That would have been a realistic reaction. Now, I don't know how the CVs will get back Narmada accepted to the audience, after making her stoop lower than Aparna (if at all the precap is not anyone's dream).
Having said that, Narmada is not an evil saas like in all ITV shows. She is like an ink paper who absorbs whatever color thrown on her. If Arpita/Imlie fills her with bright colours, she will shine brightly. If BM/Jyo throws dark colours on her, she will come up with a dark shade. At the core, she is a nice person who is not aware of the black shades of this world and why she brought up her kids in the same way. Till Aravind's death, Aryan was this softly-spoken, witty carefree guy; but he couldn't manage his grief when his world turned upside down and had to seek the path of revenge to subside his pain. One reason is the way Narmada had brought him up. Same is the reason, why it took 4long years for Arpita to overcome her trauma. Because Narmada who herself had turned a blind eye to the harsh realities of life hadn't prepared her kids to face the same. For a mother like Narmada, as long as she can see happiness on her kids faces nothing in this world bothers her. Even if you say the sky is falling down, she will be cool as cucumber if her kids are happy. But the moment her happy environment is disrupted, she becomes a completely different person whose overprotective mode turns on. And that is what makes her behave irrationally.