So in regards to the whole Olga Marie being>! Beast 7 !<here are my two cents.

1. Beast VII is using Olga Marie body as a vessel. The actual Olga that the MC and Mash oh so much love and adore is the Alien Priestess. Some of y'all might ask "but but porque?" reason i think so is because once upon a time in a burning land called fuyuki our good ol chap Lev told Olga that she couldn't go back to chaldea since she "lost her body to the explosion" Now if Solomon has taught us anything about the Beast class its that they can apparently inhabit dead bodies and use em how they want. SOoooo it wouldn't surprise me if Alien God aka Beast VII found said corpse and was like "oh hey look free body"
2. Why i think the alien priestess is Olga Marie? Lev did mention to Olga as she was getting sucked into Chaldeas, that Chaldeas was sort of another dimension. From there i wouldn't be surprised if either A. Beast VII found her and turned her into the priestess or B. The alien priestess found her, took pity on her and absorbed what was left of Olga into her, either way it would explain why she kept appearing in front of MC and Mash and show some sort connection or reaction to them. I mean hey Beast VII while in Olga body was like "ummm who are you guys" when MC asked what "Olga" was doing
   

But then again i could be wrong so who knows ¯\_(ツ)_/¯