Hi,
I think the new Mihir is Nandini and Karan's son, who was kidnapped by Mandira when he was born. As Mandira will be reappearing soon, it might be like this. Mandira want to take revenge on Tulsi, as she considers Tulsi that stealed away her son Karan from her. She has perhaps instigated Karan's son against Virani family. Another possibility is that Mandira gave KN's son the name of Mihir Virani (Junior). Remember Nandini said that its possible there are two person with same name. Another point is that this new Mihir seems to be very young with black hair, compared to Mihir Dadu.
This is only my thinking.