Maybe it wasn't just Sarrissa, maybe it started long before Sarrissa was even born. We don't have enough information (also note that Sarrissa is her daughter. Perhaps that aided as well). I beleive it was stated that Mab has been Winter Queen for a thousand years?
Why does she have to want it?
How do you explain the clashing WoJ's and MW's statement?
How do you explain the WoJ?
Previous WoJ stated that Mab have no soul, that she is unchangeable. After CD, I think it is possible to question the validity of that particular WoJ. When text and Woj contradict each other, I usually side with the text. WoJ is just Jim's ideas that is not yet put into cannon. He have the right to changed his mind. What is absolutely cannot happened is text contradict text. That will show that Jim is ill prepared. Jim has never make this mistake before and I hope he never will.
So, it is possible, just possible mind you, that Mab can be changed. Maybe, it is even possible to rekindle her soul. According to Mother summer, change in immortals did happened and as we can see with Odin/Santa, such evolution did happened, but it takes a lot of time. Is Odin transforming into Santa is a soul changed or a mantle change is anyone's guess but change in immortals did happened, that is for sure.
I expect that Mab have to be willing to change. Changing a vanilla mortal without his/her consent is already hard. Trying to changed Mab against her wil will be impossible. I don't expect the rules in the DV is not to much different from the real world in this regard, so it is just simple sense.
I did not believe however that just Sarrissa and Harry is enough to changed Mab, not in such short a time and with so little interaction. Especially if Mab don't really want it. She told Maeve that fulfilling your purpose is not slavery, so I assume she is quite satisfied with her current circumstances.