Could the secret be as simple as, Gauri's mom was very rich and her mother big tikka woman did not want her to marry a guy like Alok because he was poor so she disowned her daughter.
I think Alok is just afraid of her because she disowned his wife and hates his guts. So he probably is just afraid of the confrontation, and he is afraid of what Gauri might say if she knew that her parents got married under strained circumstances.
Mahen knows the whole story. That is why he got Shiv and Gauri married to claim Gauri's property on Shiv's name. That is why Mahen planned this whole Shashank drama because even if he managed to convince Shiv somehow before to marry Gauri and it was an happy arranged affair. The wedding would have been publicized by the media, so I think if the big tikka woman knew the truth before she would have stopped the wedding.
That is why Mahen created the Shashank dummy to make everyone think that Gauri was going to get married to Shashank, and then do a switcheroo in the last minute with Shiv. I think Shashank was trying to tell this to Gauri in that letter he wrote.
Nikia gave a huge hint in that conversation with that woman. Nikila said 'You have tried this before to break this relation but it will not happen'. How can she have possibly tried this before, if Shashank disappearing and Shiv getting married to Gauri was unforseen.
Could Nikila and Mahen have discussed Shiv's marraige with Gauri before Shashank came into Gauri's life. But they soon realized it cannot happen because they knew this old woman would stop them.
Now the tikka woman is just pissed that Mahen did all this secretively and is trying to ruin her Grand daughter's life. Because she probably knows about Mahen's greediness for money and is trying to use Gauri as a pawn. I don't think she is evil, I think she is trying to protect Gauri. Because Alok is an idiot for falling into Mahen's trap.