Mangala and harki's love for their children is incomparable.
Harki never loved kamli in the first place. It is just that Harki never hated kamli either and therefore had no reasons to ill-treat Kamli (like she ill-treated Nimboli).
Harki was tolerant of Kamli only because Kamli was her own daughter and was not a trouble child. But she never really loved her, for her love to degenerate.
She loved her Sa and their son any day more than she loved K. So when she tried to stand up for Kamli once (during marriage with Pushkar days), she immediately stepped back as soon as her Sa got angry with her.
Mothers stand up for their kids in many situations. Harki couldn't stand up even for once.
Both Harki and Anandi are mothers of convenience. Very laid back when it comes to standing up for their children. Although Harki would beat Anandi in her love for Kundan, against Anandi's love for either Shivam or Nandini or Amol.
Mangalas love hasn't degenerated, but it has changed forms.
I will compare Mangala to Waheeda Rehman of Khamoshi (as being discussed on another thread started by Leavesandwaves).
In Khamoshi, Nurse Radha (Waheeda Rehman) also falls in love with her mentally unwell patients (men) twice, which happens due to constant love, care and affection she showers on them. Even though she always knew that she is only their nurse and do not have any "rights" on her patients.
Love is a natural instinct and It doesn't happen keeping in mind the rights one has over another.
Mangala's case is very similar. She selflessly cared for this little innocent life that was brought into her world. Her love for Nimboli was a natural instinct, which got nurtured as 12 years passed by.
One fine day, this biological mother comes from nowhere and M is expected to give up her 12 years of emotions in a go? Yes, love is possessive and it also has expectations.
That is why mothers become possessive when bahus get married to their sons, or women become jealous of other women getting close to her man, or a child is too possessive to share it's mother with other children or even siblings (in some cases.)
Mangala's love for Nimboli was / is genuine and definitely not comparable to Harki's love for kamli