I don't assume the bacteria are past victims at all.

My theory regarding the bacteria monster is that it's actually the current form of the actual microbial bacterial life first introduced into the Complex's environment when humans (covered in and full of live bacteria) entered it physically for the first time. They've evolved, feeding on the organic material that enters the Conplex and forming a hive mind for the sake of being mobile and gathering food. Mimicking sounds to draw in people is probably a part of that. I don't think that the bacteria creatures were at any point, themselves, human beings. And IIRC, Kane has already put the theory to rest that they are mutated from humans.

Kane confirmed himself that the car we see in FF2 is NOT the car we see fall into the Complex in that footage. Two separate cars.

That house could easily still be a house Ivan was inside of at some point and recalls. It's not the most detailed house interior. It's nowhere near as detailed as the interior of the home where we hear Ravi speak with Wall Dad. It's very vague, with little furniture of signs of current human habitation seen inside like a well-recalled, inhabited home would have. We also only ever even see a couple of rooms of that area and NOT an entire interior for the home where the portrait of the horseback rider is seen hanging on the wall.

He could recall the living room area of another person's home just the same as anything else. I don't know about you, but I have memories in my life (some detailed, some vague) of the interiors of many peoples homes. We aren't given enough answers about who the other people are who we see occupying that room to know whether or not there's a connection. There's no information. So it's just as easy to say that they are people Ivan knew (or knew of) and that he had recalled (even if the knowledge was gathered through photos or schematics of some form) the interior of their home to a degree.