Short answer, Mohan needs money. Kastur's brother and his father-in-law agreed to finance Mohan's business venture. He plans to use that money to pay off the loan sharks that are after him.
Saroj never liked the Galas. Also Kastur has only passed 9th standard, and Saroj knows Mohan is marrying for money (but she doesn't know the extent or what exactly his troubles are) and asked him not to, and even tried to get him out of it. To no avail.
Plus there were a few misunderstandings between Saroj and Kastur, which only further increased and Saroj's dislike.
As for whether Mohan is acting--depends on specifics. Yes and No. No one knows all the mess he's in. And he feels guilty about Marrying Kastur for this especially when her family praises him. But I think he's accepted that Kastur's going to be his wife.