Balakumaran didn't tell it clearly. That painting is not in Chidambaram.
In Periya kovil, around the karuvarai (in thiruchutru naazhi), lot of paintings are there. Exactly backside of the lingam, there is a painting having Chidambaram temple (pon vimaanam is the evidence) and a man with beard worshipping natarajar with his wives. That man may be Rajaraja.
The reason Rajaraja married vanathi may be pure political as a thanks giving activity to Kodumbalur velir. And she died after Rajendran's birth and before Arulmozhi becomes the king. Kalki gives importance to every character in each scene and the characters also impressed the readers. To give an opportunity to vanathi to impress the readers, kalki placed that scene which will have direct link with her death.
Even though poonguzhali is a fictitious character, kalki had 'Panchavan madhevi' in his mind while creating poo's character.