I mean from the very beginning it has been very clearly established that arjun had immense affection for Pooja as his Bhabhi and nothing else.
Also, the reason he went looking for the diary is because he thinks his father is trying to hide something and wants to unearth the reason his father would even bother to hide something, if nothing was wrong.
Also, Pooja and Yash lying to their respective spouses and meeting each other, dying holding each other's hands just doesn't add up if Pooja and Arjun were
If Pooja was having an affair with arjun, then Yash was innocent and then Zoya will never ever move on and Adi will never trust anyone ever in his life again. having an affair
The whole purpose and basic plot of what story will be ruined.
These are my two cents, do share yours