I think the simplest explanation would be that the cvs didn't realise till today that the audiences dislike kamla and woke up to the fact when they probably got a deluge of angry reaction from them!
Just kidding.
I think all those flashbacks were shown because Kamla, with her tiny brain, felt that since it was Pakhi who confessed her love for Raghav first and that too so early in the story, Pakhi had a right to Raghav. Plus, she must have felt that Raghav may have felt the same.
Though why she should persist with the thought when she saw how happy Raghav was when the rings were being exchanged, beats me. Also, shouldn't she be berating Raghav for 'two timing' and trying ruin Kalpi;s life instead of slapping her own daughter black and blue?
I think her attitude towards Kalpi over the years is a mixture of several things..you've touched upon many yourself. Her gratitude towards the kapoors, including Pakhi, for employing her so that she could educate her daughter(if anyone should feel left out here, it's poor Pakhiya!). This meant she had to neglect Kalpi a little. Also, a mother is usually a little stricter towards her own child than to another's...here, I agree, it is to make your child better behaved and grounded. Another factor could be that she feels Pakhi needs her more as the girl didn't get what she needed from her parents and brother while Kalpi had a pretty strong base there from her dad and brother.
And, as someone has already said, she might be burdened by the servile mentality...towards the kapoors.
Finally, if she weren't always portrayed as saint, I would have had the nasty feeling that she was secretly pleased and thrilled that Pakhi prefers her to Neetu. Who, btw, told Pakhi that it was Kamla who breast-fed her and not Neetu? That seems to be the basis of Pakhi's resentment towards her mom.