Well, I was absolutely livid when I saw what they showed us on Jan 3. I thought how could they show such a ridiculous way of the truth coming out. But then again going over it again and again I realised that what they showed was fairly logical - that is if you overlook some glaring bloopers such as
-Jai introducing Anupam Kapadia to Bani when it was he who had changed the fatehr's name to Anupam
- Anupam Kapadia helping Pia with her drama
But putting myself in the creatives shoes this is what I think:
When Pia came back and realised that Jai had not told the truth to Bani and Rano was very angry with her, she knew that she can be sure that Bani would leave WM only if she is convinced that the baby she is carrying in Jai's. If Bani thought that she had a one night fling, she may forgive Jai and continue to live with him. That is why she did not bother showing that CD. Moreover that CD showed that it was Pia who was seducing Jai. That would only invite Bani's wrath on her and not Jai.
But if Bani knew that a child was involved she will definitely go. If Pia were to tell that it is Jai's child, Bani may or may not believe it since she is obviously so in love with him - not everyone is as gullible as Jai. So the best way to get the truth out is for Jai to admit it himself. Now Jai will not do it on his own. She realised that he has tampered with the hospital records to cover up his sin. So she used his own lie to betray him. Now the truth is out of the horse's mouth. Bani's anger if any will be directed towards Jai rather than her.
So one has to grant it to her. She does have a master brain. Only all that intelligence has been put to crooked use. No wonder she was nervous when Pushy took her for the DNA test. I'm sure the results will show that he is the real father. If that truth comes out before her drama then she cannot be sure Bani will leave.
By the time the DNA results are out Bani would have left WM - hopefully to become a much stronger individual.