For those interested here is the link to the Review of Evidence that was posted before the spoilers were released.
Clues from Spoiler
Just as an aside:
In my knowledge as per Indian law SP's former mistress's son has equal rights over familial inheritance.
However if as I suspect SP is noveau riche and has earned all the money then he can write a will and is free to distribute his wealth as he pleases - percluding the illegit. So the former mistress's son can end up being a loser. The mistress can lay no claim to familial inheritance or SP's wealth.
Now to get to my point.
IMO the reason for the child swap was for money - SP's wealth.
Now after so many years - G3 has grown attached to Yash and SP not only is attached but dependent on Yash with respect to his business. In other words, no way will G3 or SP think of cutting Yash off their will.
And when the truth of the baby swap is out - SP will be emotionally obligated to include Akash in his will. Besides G3 will want him to as Akash is her biological son!
The baby swap was calculative not a spur of the moment revengeful act.
Similarly IMO Ansh's kidnap is calculative not a revengeful ploy to separate AarYa and get Yash remarried. The initial abduction of Ansh from the hotel was revengeful, the ransom too was revengeful but the person who planted Akash was calculative and was waiting patiently for this moment when there would be a chink in SP's armour.
Akash was chosen and planted as the accomplice by a quick thinking and opportunistic person or persons who had been on the look out for the right moment to strike at SP. It just so happened that both opportundity and the circumstances under which SP was operating worked in their favor. It was Yash's kid who was kidnapped and the revengeful terms of the ransom required Yash to separate from a wife that he loves dearly and remarry - this was a golden opportunity that the calculative person/s perhaps never expected.
A golden opportunity where Yash could be given a counter ransom demanding that Yash and Akash swap places as groom-to-be. This way the calculative person/s uses the wedding mandap to declare Akash's legitimate status and Yash's illegitimacy while returning Ansh and Aarti back to Yash!
Bottomline: SP and Bua (perhaps Pankaj?) were behind the revengeful plot and may be SP's former mistress and Akash (perhaps another person?) were the calculative person/s who were waiting to strike at SP. The reason is to humiliate SP while making sure that both the legitimate and illegitimate sons (Yash and Akash) can lay claim to their rightful inheritance!
So dear friends IMO - the motive is money. One of yesterday's spoilers indicated that SP's former mistress lives in a chawl. So it struck me that her insecurity would have been tremendous when SP dumped her - both for herself and the child in her womb. I assume her personality type is that of a grabber - a person who will grab her rights come what way. She may be brash and crude in speech, lacking class - but she will definitely be mentally sharp, calculative and will want her dues (money and claim for respect). She was waiting for the right moment to strike and fortunately or unfortunately SP played right into her hands!
What I have not been to tie as yet the means - that is how Akash was planted. Of course it is the Mangalpur connection - but the details have not been shown by the CVs. If that one piece of the jigsaw is revealed - the rest fits in well.
Now the calculative person/s (Akash, SP's former mistress, and perhaps?) will be awaiting Ansh's gift and Yash to show up in order to place their ransom demand which is what Yash (the mistress's biological son) desires - Ansh and Aarti for allowing Akash to be substituted in his place at the wedding mandap!