Who is Mrs. Flood?

Eric's had a Doctor Who / Mrs. Flood theory we've been trying to release through @theolddoctorwhoshow for the last few days, but thanks to numerous technical issues we weren't able to post it until now. 13 hrs ago, Screenrant got there first. Or at least a part of it. It doesn't matter who got where first, right? Only the friends we make along the way? Who IS Mrs. Flood? And how does she relate to the Doctor and Ruby? Do you think we got it right? What do you think? Leave your own in the comments.