Then why is it complicated? Can we just dismiss it as "twisted TV serial (il)logic"?
No. I think there is another way of looking at it.
The point is that all human beings and relationships change with time. The Arjun and Purvi who first fell in love with each other were different from the Arjun and Purvi we see today. Both of them have been through a lot of experiences, good, bad and ugly. Their respective spouses, Ovi and Onir, have been integral parts of these experiences. Even if Arjun and Purvi were to get married today, they would never be able to erase the impact of these experiences.
That's why, I think, it makes sense if Purvi goes back to Onir and Arjun patches up with Ovi.