Regan, the human

Of course she could survive. She was kept alive by a battery of medications; the physical pain was probably only felt in the beginning stages of the full possession; the room was only deep-frozen in the film, not the book, and even so, it was not enough to kill the priests, Chris, Sharon, Karl, the doctors, or anyone else - plus the film only shows deep-frozen temperatures in the relatively brief exorcism sessions. When Karras visits her room, nothing is frozen whatsoever; the crucifix-rape wounds would have been very easily treated at home, or Dr. Klein could have been brought in and would have been told - and would have agreed that - the injuries were self-inflicted as an effect of Regan's "mental illness"; the 180 degree head turn would not necessarily have killed Regan - I've seen this acrobatic trick done you You Tube - the full 360 degree turn would have killed Regan - HOWEVER, it is very likely that BOTH head turns are illusions performed by the demon on the already shocked-and-rattled Chris, and on the stunned and highly-stressed priests.

Regan was a fighter and a survivor.

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I treat the 360 degree head turning as pure illusion construct from the demon.

The 2016 Exorcist TV series has a better take on the head turning issue. In the series, a 180 degree turn is the method of execution the demon uses to kill his victims. I thought that was an interesting concept.

The other injuries to Regan are survivable.