Mahidevran letting Nergisşah go to live her own life in E129 is an awesome and significant character moment. It shows that she has grown enough to let go of a person of unconditional and actually understanding support she would otherwise crave back in S01, back in the more "low-scale" and personal circumstances where she would constantly tell the people she relied on to help her, protect her, be there for her, to not leave her alone. Now that she has gotten over the former pain and has to deal with a new, even more hard hitting one, along with very unfavorable outward circumstances where she has debts and it's possible for her to not manage it at all, this is not present anymore. She's totally willing to put herself and her own feelings of resentment behind in order for her granddaughter to have a new, better and happier life and not stay and "wither" with her, no matter how much Nergisşah wanted to. Despite of the marriage being arranged by Sultan Süleiman, it's not really about going because it's an order by him. Mahidevran already follows the orders solely for a personal drive and motive, not because she's obliged to, as seen in E124. She has already come to terms with what's happening and wants Nergis to embrace the chance of a better future. She doesn't want for her to forget her loved ones (telling her to name her future son after her father), but is still going to let her progress regardless, not put her on a standstill to "sink in this mourning" in order to have someone to talk with or vent in front of. It certainly might have been a hard decision to make, judging by the sad look she gave Fidan when Nergisşah expressed her objection and the moment of silence before her response - it's not easy to let go of someone you love and that loves you so dearly. But Mahidevran has already gained the inner will and strength to handle this, wishing the best for her support network from Manisa that truly helped her rise up from the ashes of the harem's toxicity and gain more agency.
The same goes for Mahidevran wanting to let Fidan go, too, a bit later, which amounts to her refusing the ways of the system. While the other servants left because they had to go with Nergisşah, Mahidevran could've easily not made the offer to Fidan to go as well, for her to have at least one more pair of eyes and ears to listen and help, but she made it anyway. And she let her stay not because she wanted her near her at all costs, but because such amount of loyalty wasn't something she expected in that moment. It could even be paralleled to Hürrem and Sümbül in some way.
