I agree. Suicide is not reasonable in my opinion. Women after a traumatic rape don't typically immediately commit suicide, perhaps days, weeks, or months later. But not immediately after. In that moment you're in survival mode. My thought is, had she been conscious and alive, she would have left the park and scene of the attack - not went to the waters edge and passed out.
I discussed this with my sister who is a traumatic rape survivor (similar situation - she had also been drinking to same extent as Libby, was similar age, and it was mid winter and snowy) to help me understand frame of mind in such a terrifying time.
Yes I think neither scenario makes sense
If suicide - then likely not immediate and you'd chose a better spot. Throwing yourself in that part of the river doesn't really suggest a quick and painless death.
If accident, then why wouldn't you just get out?