Okay my analysis, I might be completely wrong as its mystery thriller,so something might be happening completely opposite but ya let me put my thoughts:
I think as ADI-pooja relationship was lossening up as we can see in last epi,coz of adi's childish nature n friendship zone n no depth,I think she started to emotionally connect with yash coz of same thinking ,in depth thoughts n like hobbies which might have pooja might have found solace in ,she started to have emotional connect,might have started to get attracted to yash or might be just a emotional supporter or temperory attraction to the emotional thing they had,but soon she must have realized its just that the share emotional bond but nothing else so might have backed out n be just be friends with him...than Arjun angle,he might have been had affection n soft corner for pooja since childhood(third person in trio which usually have)but once adi-pooja got into relation he might have lost his chance(also emotional baggage even family concentrates n loves aditya more than him even when hes wrong or not the deserving one-like mostly happens in sibling-papa mummy bade beta/beti ko jyada pyaar karte hai) bt later he might have got as an obsessive lover kind of feel for pooja..once pooja got to know,as the spoiler said yash started to help pooja to get out of this mess..arjun pooja might not had anything ,but Arjun was obsessed with her...pooja didn't say this to adi coz he might have been completely shattered knowing this(already he has been through so much in family)than regarding pooja being pregnant may be she didn't wanted (her n adi's) child now as he himself is baby stil...
So I think pooja had thing for yash...yash was like a friend to her(but might be emotionally carried away ,but didn't cheat)n the accident part ,zoya s dad knew yash was having an affair ,n to take him off from zo's life n prevent his daughter from this cheating ,he was the one involved in getting that accident cum murder...n adi's dad knew pooja is having an affair but with whom he might not be knowing...
N so Arjun is behind diary coz pooja might have included his name as stalker in her life,,coz he was like smitten reading the diary...n mr.hooda as he was alredy knowing about affair n diary being public will affect his image n all would know he also knew about it...n pooja's mom had hint pooja wasn't happy in marriage n something was bothering in her life but wasn't having an idea about what..
So adi is gotten cheated ..with time he moves on,zoya would know what she lacked in giving in relationship that is communication...both moving on from one being cheated n other loosing spouse..both having emotional connect coz of same pain,n both believe in sincere n loyalty,zoya is epitome of sincere n loyalty which adi wanted in his love life,n zoya getting a partner who lifts her up helps built her confident n fighting the cruel world by being dhal not babysitting her...both with same grief of destroyed perfect life and zoya able to read adi's state..both become friends n slowly fall in love coz of good emotional connection(with strong zoya and matured adi)..but than society would still believe their partners had an affair n now they r having affair with constant closeness in events n mahi's photos..so this war against society n love norms,,they defeat this n live happily ever after giving love their second chance n keeping their past in heart n memories n present with their fate brought partners..