In those few moments, Mandavi showed that she would be a better queen than her mother in law. Even when she saw manthara busy in praising Kaikeyi and conspiring, she didn't wait to eves drop, (any normal girl would have done that and would have ran back to the others to reveal the factor that it was manthara who is behind the changed Kaikeyi. Instead she just announced her presence to the duo by calling maa and kaikeyi was confused to see her there at that moment while Manthara was danced to her calling maharani Mandavi. In her peculiar happiness of winning over Dasharath's plan to crown Ram as the king, she forgot that Mandavi had primanded her for talking so. Mandavi was clean and clear and conveyed her message crystal clear that her husband will never be happy with what his mother had done to him (his relation with his family has been chopped off) by asking the crown for him which was not in his right. She gave a clear message to her mother in law that she doesn't want others to poke fingers at her husband for her brother in law's exile. In other words, she told her MIL that she will be held responsible for what ever is happening in Ayodhya and not her husband. What a way to convey the truth. Mata Gargi will be happy to have got such a student.
And one thing I would like to say here that Kaikeyi never had spent time with Bharath to know his nature and likings. Initially he had lived in Gurukul and whatever limited time he was in palace, she was busy with praising Ram and belittling Bharath by comparing him with Ram. She never showered her love on him and the moment she came to know about the promise made by husband to her father, she sent Bharath away to Kaikeya to make him able king. later after his arrival at Ayodhya, he went to Mithila and got married to Mandavi and ever since then, it was Mandavi who was in his company and not Kaikeyi. Her soothing factor is just that her son once had said he will choose her if a time arises where he has to choose between his brother and mother.