I would have also been angry but thing is that she was waiting for Malhar to say that it is not true but he also admitted to everything in that letter so it's not a matter of trusting that letter. Letter was just a way of reveal it to her. Not like Malhar was denying it and she still believed the letter.
Now I feel 100% sure that he will come and offer Kalyani the same deal he offered Malhar, but Kalyani not hesitate to take the deal for Pillu. We'll like to see the crazy, passionate and possessive side of Malhar now. But when Kalyani goes fo his house, he'll explain his fake backstory and show her the portrait of Kalyani look alike and explain why he asked for her. So intially he'll look like a villain to Malhar, then he'll reunite them and gain KalMa's trust and then betray them. And during this MU/anger phase, Malhar will run after Kalyani and AS will try to get Kalyani and Trilok married.
Also, I think Malhar knew that this wouldn't be easy and Kalyani would be EXTREMELY angry that's why he said, if you can, please forgive me. He's expecting this, so he won't give up that easily, especially with a creepy new guy after Kalyani.
Makes lots of sense.... he'll play well-wisher first and then stab them in the back and then we'll learn what his real motives and agenda against Malhar is.