Advait does occasionally keep things from Shamsher. So, it's possible that this marriage is also something Advait hid from Shamsher. Also, Harleen was concerned about coming in contact with Advait, but she never really cared about Shamsher. That is why I assumed that Shamsher is unaware of Advait-Harleen marriage.
@red: After returning to KH, I don't believe Nehmat will make an effort to meet Ekam. Therefore, I think her decision to return to KH was influenced by this Golgappa scene.
What advait done in past that he didn't hesitate to marry harleen to save himself and shamsher didn't know about marriage but can get resemblance of harleen with jass