Here is what I think will happen. Supriya would trick Naku and run. At home, while everyone's worrying about their delay, Kala would take Aayisahib to Supriya's room and show her the letter along with a fake show of concern. Later, Naku will return and tell Aayisahib what has happened. Now, here is the crux. In spite of the letter and an outraged Kala, Aayisahib would ask Naku to step in as the bride(Naku of course having no idea about the letter). Why would Aayisahib do it? She'll do it only if she is the proverbial 'badi hasti'.
Look, the badi hasti is smarter than Kala. She roped Kala into her plan to help Supriya out, because she didn't want to disclose herself. It is infact a much more neat and safe way to stall the wedding, rather than bumping the bride off! Aayisahib could be thinking that it is a better way to ensure that Dutta will never marry again i.e. by burdening him with an ugly wife whom he could never love(or so she thinks) nor trust. To make sure of it, she would let Dutta find out about the letter after the wedding.
what do you think of it?