Another car hit her from behind but the insurance company said her car was already totaled from the first impact so didn't assign any fault to the car behind her. (I don't totally understand that part.)
I assume that the two impacts did not happen simultaneously. If so, since the insurer (not sure which insurer you're talking about) determined that the car was totalled by the first impact, then the second impact caused no damage (since there's no such thing as "uber-totalled" or whatever).
this was basically what you call an accident. Stuff happens when a bunch of cars are all in the same place at the same time on a public road, and this was one of those things. There wasn't anyone doing anything horribly wrong, it just didn't work out well.
Collisions result either from negligence or intentional acts. Describing a collision as an "accident" simply means it did not result from an intentional act, which means it resulted from the negligence of one or more of the drivers involved. Thus, I'm not really sure what point you're trying to make here. In the case of a rear end collision, the driver in the rear is typically negligent because he/she was following too closely to stop safely.
I didn't understand why it mattered what Medicare will or won't pay for. Isn't her primary claim to the insurance company, not Medicare?
Obviously we have no way of knowing anything about your mother's eligibility for medicare or whether medicare would or wouldn't properly cover anything here. As far as insurance coverage, I'm not sure whether you're talking about your mother's auto policy or her personal medical policy. If she had medical payments coverage on her auto policy, then that will pay for medical expenses, but medpay coverage is typically very low (e.g., $1k +/-), and you described injuries that would have gone way beyond that. If you're talking about a personal medical policy, we obviously have no idea what sort of coverage she had and how that coverage might interplay with any medicare coverage.
Are these because she was 51% at fault and the other driver was only 49% fault?
Neither medicare nor medpay coverage on a personal auto policy nor personal medical coverage take "fault" into account.
I am very confused about who is supposed to be paying for what.
Submit the bills to everyone and let them sort it out. Better yet, provide all relevant insurance info to the providers and ask them to bill as appropriate.
Shouldn't some combo of insurance and medicare be covering most or all of her bills?
Again, we have no idea what coverage she has, but insurance isn't going to apply magically if she's paying the bills out of pocket.