If Lalit Juneja gets Prem back

Why does he want to break the Prem Heer jodi, even if he gets Prem back, ash will not agree to marry him anyway, so might as well get him married to the love of his life, or will he start looking for another girl for him?? With Vishaka already married, and Ash gone, there is nobody left in the DJ clan anyway...

I agree.Why can't he accept heer now?

I don't think that its really a matter of accepting Heer. Because Lalit had so much pride in his son, Prem. Its kind of like putting all of your eggs in one basket. You know? You shouldn't do that, because they will break...
Lalit did that with Prem, he thought that Prem will do whatever Lalit said with his eyes closed.

At the wedding, that didn't happen. Hence Lalit took a beating to his ego and pride, which relied heavily on Prem.

More so than that, I think that he (Lalit) really blames Heer for the way Prem betrayed him. Reason being, he asks Heer to return him his son. As if she had bought him over or something, like she was holding the reigns of the situation.

I just hope the fact that Heer is Balraj's daughter comes out soon and Daljeet is History...