I am not here to bash nor to take sides. I am here to give an analysis of what has happened in this triangle, starting from the time Nandani went into coma.
Tanya knowingly married Karan when he was married to Nandani who was in a coma after giving birth to Bhoomi. She is his best friend and she secretly loves him. She was by his side, seeing him constantly pine and cry for Nandani, repeatedly. Because she loved him, she turned good and testified against Ansh to help Nandani get a divorce. When Karan found out that Tanya helped Ansh, he was angry but Nandani convinced him to forgive her.
She raised Bhoomi and tried to go away once Nandani came back. Even though she told Karan that she is willing to share him with Nandani, Karan told her he is helpless and cannot loose Nandani. She had no choice but to leave. When she left, she met Bhoomi secretly and eventually Karan. They met behind Nandani's back. Then when Karan was put into jail, she didn't hesitate to tell Nandani's doctor that she is pregnant nor did she hesitate to come meet Karan in jail when Nandani wouldn't be there.
As a result, Nandani found them together and overheard that they are married. Broken and realized she's lost everything, she freed Karan and accepted the blame. She put their hands together and never once expressed to them both how hurt she is over their betrayal. In fact, she thanked Tanya for everything and left. Then, she never came back to Karan after her release.
20 years later, Karan-Tanya are together but aren't happy. Why aren't they happy? Because Karan still harbors the same feelings for Nandani as he did 23 years ago and as a result, he gave Tanya the status as his wife but not his heart and soul, which is solely Nandani's.
20 years before, Karan was confused all the time. Now, present time in the serial, he is very determined. For 20 years, without Tulsi and without Nandani, changed him to become who he is: shrewd and to some extent selfish.
Now, not even Tulsi can change his mind when his mind is made up. He was living along life lonely, one can say, despite having everything. Tanya too is lonely as she doesn't have Karan's love but the status as his wife. In conversations between them, its obvious that Tanya has told Karan that she will leave once Nandani is back.
Now Nandani is back. In her explanation to Karan, she told him that she cannot ruin Tanya's home by coming back to him and clearly told him to leave things the way they are. She is doing her part as a friend, so she cannot be blamed for anything, as she has ran away from Karan so Tanya's home won't be broken. Where was she all these years, doesn't matter as what matters is the reason WHY she didn't come back.
But Karan has always loved Nandani only. Yet he realizes that Tanya has done a lot for him as a friend and as a wife. Sometime back, he did love her but not as much as he loves Nandani. And that love doesn't exist anymore as well because it wasn't as strong as his love for Nandani.
In life, we fall in and out of love many times, because the people we fall in love with, impact our lives in many ways. But the person whom you love the most is the person for whome you slowly die when you are living without them. And then are willing to have them, no matter what you have to give up to have them. Same thing happened with karan.
WIth Nandani, it was pure, genuine love that stood the time of Ansh, his acceptance of Bhoomi, and her coma time, and now 20 years later, his love is still the same burning flame. He cheated her, he betrayed her as he felt torn about what was right to do. But he still loves her, he pines for her, and cried every night without her.
With Tanya, he respects her as a friend and accepts that she did a lot for him. He gave her the status of his wife because it was her wish to be living in SN as his wife and to raise his children. But he cannot simply love her. He did once because at that time, he was urged by Tulsi to move on. He moved on but only to Nandani come back to him from coma. He tried to move on again when Nandani went to jail and told him to take Tanya's hand in his. But once Nandani didn't come back to him, he became shrewd and hard-hearted against anyone but Baa and Bhoomi.
Now, he realizes his happiness and love is Nandani. But he still is concerned for Tanya. But again, its a matter of happiness verusus duty. For the past 20 years, he gave Tanya her rightful place in SN as his wife but couldn't make her happy nor himself.
Now, if Tanya moves on without any kind of drama and puts Nandani's hand in Karan's in front of everyone, the way Nandani did 20 years ago, Tanya is truly maahan. If she doesn't, then I cannot say anything good about her.
For me to accept that she is a good person would mean that she would have to do the same thing Nandani did:
Happy put Nandani's hand in Karan's in front of the children and members of Viranis and permanently leave.
She can still find someone to marry, look at Parvarti from KGGK, she re-married.
Manthan is right in his own way but he shouldn't have been very mean regarding Nandani. I don't see how its Nandani's fault that Karan and Tanya are willing to divorce each other now. Because Nandani doesn't want to come back to them nor did she ask for her right as Karan's wife. I think he's a better son than Nakul and Lakshya who cannot do anything worthwhile.
Bhoomi is also right in her own way. She has seen Karan's and Nandani's tears for each other. She sees their pain in their eyes. She has witnessed Nandani's explanation, so she is right when she is supporting Nandani. She is wrong though for not realizing that Tanya is her mother too. But again, she is emotional and vulnerable because she has seen the tears and pains of Karan-Nandani. She is a good daughter and I am happy that Karan does have a support in the family, as he needs it.
Either way anyone sees it, Nandani is Karan's first wife. She stayed away because she was told that when she came back from coma, she ruined Karan-Tanya's marriage and took Bhoomi's maa away from her. So for Bhoomi and Karan's sake and for the happiness of Tanya, she left them.
Again, my post isn't to bash anyone. I am just writing what I have seen so far of this analysis. Yes Karan-Nandani are married in real-life but because of their characters in the serial, they do deserve to be with each other. Tanya should realize that if 20 years with Karan cannot change his feelings for her, then even now, after meeting Nandani, will not change his mind.
She has taken the right step but let's see what happens. Again if she puts Karan's hand in Nandani's in front of every member of SN present, then I will say she is a good person.
Humaira