I completely agree about the part where one should not expect DM to forget Subhi in an instant just because he married Santu. As you have mentioned, they have a history - around 20 years of marraige and 5 children (though I still have my doubts about Maulik being his son).In fact seeing his love and devotion for Subhi I am sure she will always stay in some corner of his heart even after he accepts Santu as his wife. Also, I don't think Santu will have any issues of jealousy or otherwise with that as she herself acknowledges Subhi by doing 'pranam' to her portrait and referring to her as 'maalkin'.
What I do not like is though the marraige was one of 'majboori' on both their parts, the initiative did come from DM after being brainwashed by his Guruji. The very least he could have done was see that she had her basic needs of food, clothing and a proper place to live in being met. That store room is horrnedous with no lighting, mosquitos and rats. She did not even have a proper mattress to sleep on. Also, if DM did not want to acknowledge her presence or be bothered with her then he should not have used her for public appearances at all. Megs-Rasik are old friends - why did he have to lie to them and use Santu just so Kaddu's rishta is not jeopardised? He could have come clean with his friends na? What if they come to know later on that DM lied to them and was playing a game of pretend with them? And not once did he think how Santu would be feeling being used like this. For him it was 'chit bhi meri, pat me meri' - heads I win, Tails you lose'.