I hope the next episode will acknowledge that Sarang putting kuṅkū at Mukta's hairline isn't a legal marriage ceremony.
It's not legal because Mukta did not consent to marry Sarang. Even if she accepts his gesture and considers herself married to him, the law would only recognize it as a marriage ceremony if it was in accordance with local custom or either partner's tradition, and it was not. Nobody in attendance is wearing kuṅkū at her hairline. It's not a Marathi wedding tradition.
It's copied from the Bengali original, in which Shongeet made an even bigger mess of sindūra falling all over Jomuna's forehead and nose. Applying sindūra after Saptapadī is a Bengali marriage ritual.
I guess Sarang had to do something irrevocable to demonstrate his commitment. Saptapadī would take too long ... he didn't have a maṅgaḷasūtra handy ... he could have simply taken her right hand in his and declared it pāṇigrahaṇa ... or applying a dot of kuṅkū at the middle of Mukta's forehead would have been a more dignified way to demand recognition as her husband.
If I had to write this plot point, I would have done it this way. In the earlier scene of Chandrakala urging Sarang to claim Mukta, she takes out her maṅgaḷasūtra and tells him that his grandfather also had to make a difficult decision. Nānāsāheba Nagarkar had respectability and Chandrakala Imampurkar was the unwed mother of his child; marrying her meant losing face, but he had the courage to bring her into his society. Chandrakala puts the maṅgaḷasūtra in Sarang's pocket to give him courage. When the moment comes to put the ring on Arya's finger, Sarang nervously hides his hand in his pocket and clutches the maṅgaḷasūtra. He pulls it out, ties it on Mukta's neck, and tells her, I already followed you around a burning candle, pledging my love to you. I consider myself married to you, and if you accept, we'll get new vāṭyā for this maṅgaḷasūtra and make it legal.
Whatever gesture Sarang used, I guess the point is that he compromised Mukta's reputation, and now he can make the moral argument that marriage is the only remedy.