There is a difference between the two situations.
Abhimanyu and Akshara's marriage had ended on a bad note, both of them were angry, upset and in a fit of rage Abhimanyu threw Akshara out of his life and divorced her, so Akshara having any respect or love for him is out of the box.
But incase of Abhinav, this wasn't the case, she was happy in her marriage with him and had finally started loving him and declaring to the whole world how she would always choose Abhinav as her husband, rejecting Abhimanyu multiple times directly or indirectly. Now all of a sudden just because Abhinav is dead, and she is obsessed with giving her son a so called happy family, she is willing to move on and get married for the third time with the ex whom she never had beautiful things to tell in recent times.
The whole point of second marriage, falling in love again with Abhinav was useless when she had to ultimately go back to the toxic ex itself.
And marrying for the kid again doesn't make sense. She married Abhinav, when she was all alone, with a newborn, but now she is more independent, more settled, worldly (according to the serial atleast) and Abhir has also grown up and become mature, so I am sure she can explain her child about co-parenting and how it's not always important that both parents have to stay 24*7 together to give a happy childhood to the kid. And if the parent themselves is not happy with the partner, how can they give a happy and secure environment to the kid?