Very spoilerly spoilers.

A. Fou's intelligence is back. Very confirmed by Lostbelt 3. He's talking with a few phrases too. So there you have, one sacrifice overturned completely.
B. A mysterious figure appears each lostbelt to aid Chaldea allies when Ritsuka is not, in first two, this character introduces themselves as a "Person of Chaldea" (or "a Chaldean"). In the third lostbelt, this person was missing, but at the epilogue, the locals of a world that was completely illiterate without professionals outside the capital, speak how a "sensei with white clothes" told them it was safe to come out. Sensei a word used to address teachers, but also doctors. The description of that man is "wears filthy tattered clothes" and has their face obscured by a hood. Fans believe it's the same person because is being a "mystery."
https://www.youtube.com/watch?v=vuqYfb9kook
1:31 of Lostbelt Opening. Probably this Chaldean.
C. The woman who reaches out to Mash in the OP is also believed or speculated to be Olgamarie Animusphere, or what was left of her. She's one of the main antagonists, the "priestess of the god of another planet." Her first appearance in the game was in the prologue when Mash and Ritsuka talk about Olga, and when they rescue to the "New Director" who pleaded in a manner similar to Olga (there are parallels between some of Chaldea people, old or new, and the Crypters and Olga. They are foils).