

Mei will not sell out Hikaru
We are aware of Nasu's master plan of making Gero a harem, but why do y'all forget that Mei has been ACTIVELY opposing Nasu from the beginning? Gero wants to find someone whom he will spend the rest of his life with, and Mei promised Gero in the beginning of their deal that he will find him a wife that he will truly love and will truly love him.
Gero never wanted to find multiple partners in the first place, and Mei time and time again supports him in every way he can, saying that if Gero wants to take things into the next level, he should share it with the ONE person he truly loves.
Gero wants a happy monogamous relationship, and he has Mei's unconditional support with this, the EXACT opposite of what Nasu wants for Gero. And Mei has been doing it from the very beginning.
Of course, Mei is aware that he's opposing Nasu, and Nasu is very aware that Mei has gone astray with his original role in this whole thing, hence Mei throwing wrenches in his plans.
If anything, the latest chapter emphasizes that for ONCE in his swindling career, he will make sure that his service this time will truly help his client (Gero)... no pretending, no fake support. Because Mei truly wants what Gero wants and what's the best for it.
Gero is Mei's final act of redemption.
Which brings us to Nasu's final card, Mei himself. He will use Mei's life to threaten Gero to go with his plan. This is the reason why Mei built that wall between him and Gero, he doesn't want to deepen their relationship beyond his job as Gero's marriage advisor, and he doesn't want Nasu to have a trump card against Gero. So that when he dies, it will hurt Gero less, and by the looks of it, Mei is ready to die.