Just saw the episode again...I think you could be correct man.
*********************************Adi to Zoya during proposal dinner scene...
A : Arre dubara kar doonga yeh sab tumhare liye...mujhe tumse baat karni thi...Jis haalat mein ho na, tumhe kuch yaad nahin rahega ab...
Z: Idea...Aap likh deejiye...Likh deejiye aap...Aap likh denge, hum yaad hoga, bhool hi nahin payenge hum...
**********************************
I think Zoya won't be able to read the letter right away, because of something that comes up, so she would keep it her purse or somewhere and forget about it.
Aditya's trials continue while Arshad walks in. Aditya backs off, so as to not influence her with his feelings and giving her the space and freedom to make a decision between dosti and pyaar. However painful it is for him, he will step aside giving Zoya and Arshad space and lone time. He is suffering, feels restless but cannot leave due to his KASAM to Zoya. And that is very very painful to him but he endures it.
***********************Zoya talli and on car...
Z : Aap ko samjh aa raha hai na...Aapne hamari kasam khayi hai...Aur use tod ne ka anjaam jaante hain na?
A : Nahin todunga...
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On the other hand, Zoya will not like her time without Aditya, however good this Arshad guy is. She respects her Abbu and feels obligated to give Arshad time and a chance to know better. But at the same time, Aditya's backtracking from her life, and the change in his demeanor bothers her and that is inexplicable to her. She is confused why she isn't able to be happy with Arshad and always ALWAYS keeps missing Aditya. Once, Ammi may put some gyaan into her mind, that Aditya is her best friend and ofcourse cannot be with her her whole life, 'woh tumhara dost hai, shohar nahin na beta', confusing her even more.
***************Aditya in Zoya's bed room...
A : Agar us waqt koi mujh se kehta ki hum age chalke dost ban ne wale hain, tum mujhe apna Best friend bulane wali ho... Ek kheech ke jamake de deta usko gaal pe...
Z : Lekin hum toh aapko baar baar Best friend, Best friend , best friend bulate hain toh, aap hume marenge, marenge, marenge ?
A : Main kabhi hurt nahin karoonga...
***************************************************
So with such restless feelings in heart, the Nikah track may progress and Adi will try to remain her 'best friend', and keep her before him. And then at a very crucial juncture, the LETTER might come up again, and then she'd remember the day Aditya invited her for dinner, she is talli and she had told him to write down the important thing he had to say to her. She'll then understand, all that Aditya had endured due to her KASAM, due to his promise that'd never leave her and never hurt her.
So, I think the KASAM and the LETTER will play a vital role in the story.
#PEACE