User blog comment:IamJakuhoRaikoben/Chapter 459 & 460: Weakness & The Alighted Pegasus, Review./@comment-203.111.234.105-20151102094107

Other theory:

What if Brandish would shrink the powerful mages of FT and use them as a hostage or she would bring them to Alvarez (especially Mira since she has Seila's)

And other possibility. If mira would be captured Zeref would release the soul of Seilah to her