Based on my own experiene, all US car rentals always include liability insurance and it is not optional. Unless something has changed, collision insurance is optional but is usually provided by the credit card company, at least when a US issued card is used to pay.
I don't think having a separate liability policy will add additional protection as it probably isn't legal to make insurance claims for an auto accident from more than one insurer.
If you are injured in an accident for which you are not at fault, you can sue the other party and most likely collect damages fro ttheir insurer, based on their liability coverage, but I am not sure if, in that case, the car rental company will provide any assistance.