The Great Betrayal!

[gamesmart]

I do wonder what will they need Minfilia for in their plans.

Only thing I can guess is that Minfilia will be a type of special sacrifice or to serve as a Physical Host for Zodiark.

That's probably the most likely reason, as Iceheart has already set the precedent by using herself as a host for Saint Shiva. Both Minfilia and Iceheart have the Echo, which is likely the key in doing so.