It made sense to refer to RV as Pratap Singh's son. KN was a close friend of the father, he met RV but might not remember the name instantly, referring to the father-son relationship will jog his memory quicker. The conversation wasn't threatening to Imli but I guess she was uncomfortable about the topic. I think when she'll be suspected by Chakor/Suraj this will be part of clues against her, so this scene is for the future.
In movies and serials in my country when someone is away for a long time comes back, you delay telling them the bad news for some days until they settle down, the pregnancy is another factor. It may just be because if Chakor knew the truth she would go and see him and currently there is no actor playing the role. They can use this as a twist, Imli may tell Chakor she's talking to Vivaan or take Chakor to a trap by telling her they'll go to meet Vivaan and Chakor would believe her.
