Center is concave, a member of the mammulosa complex/

It does look weird but it's because it went 3+ months without water so it shriveled up considerably. When I started watering it again it more than doubled in size. I should have gotten a picture from when it plumped up again.

No amount of not-watering is going to make the center drop down. Or give the areoles so much room in between. Or even make it tuberculate. Or give it thick brown spines...
The eriocactus(magnifica, warasii, schumaniiana, leninghausii, nigrispina) complex always has the following:
Dense , thin almost bristle like spines and areoles, straight, untuberculated ribs, generally taller stems, and heliotropism of the plants apex.
The mammullosa-ottonis complex( most of the former notocactus), with species including scopa, buiningi, wernerii have thicker spines, concave growth centers(like most rebutia do), shorter, wider stature and most importantly tubercules.

I mean the spines on it are soft and almost hair-like and different from those on my ottonis and mammulosas. This plant came from a grower that I know almost for certain grows their plants from seed so maybe it's a hybrid?
The center also did eventually level out after I started watering again.

I looked at the top down from the plant combo. It just doesnt look like it. At that size, it would allready have dense, nearly merged spines.
I used to own a small warasii. It had already dense areoles, dehydrated and still doesnt didnt look like this.

I got more bad news btw/
Your rebutia miniscula....is also the same or similar kind of parodia....
So...not as reputable as you said ...

I guess then that it was likely a hybrid or maybe etiolated since it was inside a store. It's dead now anyways so I won't worry about it too much.

Just move it under "parodia" and leave it be then. This ...and the "rebutia miniscula kraiziana" ....

I think you're right about the "Rebutia". I'll move the pictures then.