Divine Intervention vs. Forceful Will
The first time Neel and Chahat got entangled in a marriage-like situation was when they were traveling in a bus, and some thugs were after Chahat.
At that time, though they had collided accidentally, the tikka on Neel's forehead got smudged onto Chahat's temple, creating one of the identifications in which Indian women are recognized as being married. This accident came to help protect Chahat's life, as Neel pretended to be her husband and shooed away the thugs. In this interaction, they didn't make any conscious decisions of putting on a married act. Rather, a sheer coincidence, that could be considered a divine intervention created a series of events that they used to their advantage.
The second time they got entangled in a marriage-like situation was shortly after Saraswati's death. Running high with rage, and hatred, Neel was thirsting for blood. Chahat's face, especially her temple was once again smudged with blood, taking place of the sindoor that married women wear. However, rather than outright killing her, Neel stopped upon seeing a giant bell fall onto her neck like a noose. The bell in this case parallels with the mangalsutra that is usually worn by women to identify their marital status. Once again, another divine intervention that was out of both Neel and Chahat's hands. Furthermore, Neel went along with the misunderstanding that was created by Godamvari, as a way of both furthering his own motives, but also protecting Chahat from being murdered by his family. Another parallel, or rather repetition from their first entanglement.
The third time they get involved in a marriage-like situation is when they are asked to get married according to the specific rites and customs. Neel and Chahat cleverly find a way to avoid getting married in a legal or religious manner, but they still made their own vows to each other. Neel of finding Dr. Baig and punishing him; Chahat of proving her father innocent. Interestingly, in Islamic wedding practices, it is during nikah when which the marriage contract is signed. In this practice, the bride and groom express their free will by repeating "Qubool Hai" three times. In Neel and Chahat's situation, they only made three vows to each other with full clarity and awareness which expressed their free will to each other. Neel obviously didn't say "Qubool Hai" after any of his vows, but Chahat did, which is in accordance with her religion. Moreover, this false act is another recall to the first two times they were associated so heavily with symbolisms of marriage.
Now, though they are participating in a charade, this pretense is helping to once again protect Chahat's identity and life. Moreover, they are being more mindful about each other's religions and are actively finding ways to work around each other's customs, so that there is no disrespect shown.
Of course it doesn't excuse some of the shitty things that Neel has done towards Chahat, but at least it's helping him to keep a cool head and irritate Chahat with little pranks, and not bury her alive in coffins.