I've done it! I have narrowed it down to only theory 2/2.5, 3, 4, and 6
SPOILERS!!!!!!!!!!!!!!1111!!1!1
alright, first out is theory 6, simply because we HAVE to assume the supernatural.
From reading the opening text, I find it can help support theories 2/2.5, 3, and 4
the movie Morgan rents is Ravenous, a 1999 movie set in the 1840's during the mexican-american war with supernatural themes of cannibalism, this supports theories 2/2.5 and 3, so 4 is now out
Eve's diary entries are quite difficult for me to read or gather any evidence from, however it shows how unwell she actually is and how clingy she is, we also see through checking various things as Eve that she is a murderer, and VERY observant
the current theories are 2/2.5 (vampire) and 3 (generally undead)
and yet, these are both wrong in one way or another
looking at a synopsis of Ravenous, I have come to my most likely conclusion, sometime in her life, Morgan consumed human flesh and became something inhuman, this is why we see her eating Gwen in eve's dream and the ravaged trash bags, Morgan has to eat people, this is why she is so casual about murder, because she has been killing just as much if not more, than Eve has!!! I've solved it! this is my, as of right now, FINAL theory...
I love this game, I love it when Devs leave a bunch of stuff unknown, but give you enough evidence to theorize and/or figure it out for yourself!