If Narakasura was killed by Lord Vishnu in the Krishna avatar, how was Diwali celebrated during Ramayana period?

Diwali was first started celebrating at Ramayana period.. when Rama went to 14 year exile, whole Ayodhya flung into grief . So the residents of Ayodhya never celebrated any festivals in that period . Their love for Rama was unparalleled. When Rama killed Ravana on Vijaya Dasami and started back to Ayodhaya, it took him 20 days to reach Ayodhya, since he was visiting all sages and Ashramas where he spent his exile period.. so when residents of ayodhaya knew Rama was coming back ,, they lighted ghee lamps in the whole city ,and in their houses. They celebrated it as a festival. it was a festival of light . before that day they cleaned their houses with water ( now celebrated as water filling day). Diwali comes exactly 20 days after Vijaya Dasami. I heard by a devotee (in ISKCON temple )the inner meaning of this. Our heart is like a Ayodhya , rama is lord of that city..when we remove rama from our heart ,we flung into grief . when we bring back rama into our heart, all darknees in heart is removed by the effulgence of lord.” in Krishnas time he killed narakasura on the day before Diwali. so that day also added to the Diwali list. prior to rama , in VAMANA incarnation, Vishnu sent BALI Chakravarti to paatala by putting one of his foot on bali's head . and that day falls next to Diwali day. so Diwali is remembrance of Hari's 3 incarnations.1st day- Krishna - killing of narakasura .2nd day- Rama- return from exile3rd day- Vamana - great devotion of Bali maharaja and Vamana becoming trivakrama.