Narada is an interesting chap.
Like me an hear me roar were discussing earlier.
The narada as popular perception goes was not son of Brahma. He was son of Rishi Devala.
He was not Brahmachari. His wife was panchala princess sannati. Drupad's aunt most likely. I believe he was mid generation between Vichitravirya/Prishata and Dhrit/Pandu/Drupad
Nothing much is known about his children
He was a spy and spin doctor with shifting loyalties
1. He knew Karna was Kunti's son and informed Bhishma. He was the most likely source of this information for Vyasa as well.
2. He had been tracking Karna throughout his life, which he reveals in Santi Parva to everyone, which makes me believe he had been hired for the job.
3. I dont believe that Kansa heard any Akashwani. Most likely scenario is major family heads would have started discussing that Vasudev now has better claim to throne because of his marriage to devaki and her sisters. As the child born would be Vrishni and Andhaka. And these discussions would have reached Kansa's ear. Then Narada circulates the false story of children's natural death as murder by Kansa.
4. He also then finds out and leaks the story of Kansa being a rape child. All to help Vasudev take back the crown
5. Narada is the one who tells Krishna about Pradyumna and gets Pradyumna appointed as first son of Krishna and heir of Vasudev.
5. I think Narada was Vasudev's spy and ally for a long time. And maybe Kunti reached out to Vasudev about Karna and asked him to keep an eye and ear out for him. Epic says Kunti's spies followed Karna. I dont think she had resources to arrange spies. I feel it was Vasudev who asked Narada to do the needful
Over a period of time I think Narada became too arrogant and troublesome
6. Sometime later we see Narada visiting Yudhi and getting involved in the family life of Pandavas. Telling them to make rules regarding Draupadi. This was too personal a matter for even Vyasa to talk about.
7. Narada at some point is politely chased out of Dwarka, when he similarly tries to get into personal lives of Krishna and his wives and sons. Puranas have a lot of these stories.
I think this is just before Rajasuya when Narada switches loyalties to Vyasa / Bhishma and he probably throws the Karna story in to gain their confidence.