I don't know if it's right or wrong. But I have a theory. Shahnawaz Shah wants to have it all.
Firstly he wants the party ticket. That's power.
He wants his brother back at home
Since he is a mamma's boy, he is doing everything to make his mom happy. Like both of them made a strategy to get the marriage done and have Mehrunisa in control. Since she disagreed to sign the papers. They are gonna use her for public sympathy- the prince married a common girl trope. The public is gonna get impressed because of their fake simplicity
Amal- he is used to her. She has been following him like a lovesick puppy since they were kids. He knows he can walk all over her and she is not gonna leave. That's why he is with her at the hospital, despite cheating on her all throughout their engagement. He knows he has her wrapped around his fingers. Marrying her is more of a convenience for him
MEHRUNISA is that someone who shakes Shahnawaz's arrogance and shrewdness or I must say his entire being to the core. She is unpredictable, she doesn't listen to him, and ignores him. He knows that she loves her family and will go to any extent to protect them. That's why he was hellbent to impress her family. That's the way to her heart. And he thought he's gonna convince her to sign the papers and get that Zain out. If he was going to leave her, he could've divorced her there only. Instead he whined in front of his mom about how difficult Mehrunisa is. I could somehow sense that he kinda knew she was not gonna agree to sign the papers. That gives him more time for her to stay in his life. He wants Mehru to be his life- whether she is happy or not. He wants to control her and own his enemy by marrying her. But he doesn't know what he has signed up for. She is going to eat him alive. He is definitely intrigued and attracted to her. But he won't give in to his feelings too easily. That will be stretched. Also Rafay's devdas mode is gonna help him understand his obsession with Mehrunisa and how his feelings are not just hatred, they are more deeper than that.
He is fooling everyone around him, be his mom, Mehru, Zain or Amal. Because he wants to have it all.
Someone on reddit wrote it and it makes sense esp the Amal part