Rental car not-at-fault accident help (Enterprise rip-off)?

I rented a car from Enterprise and have several questions.

-When I picked the car up, I signed a contract and agreed to a price. That price was $198. I agreed to pay an extra $111 for liability. Total, with taxes, was $303. When I went to return the car the price changed. They added a fee that was originally void and voided my discount code, so they charged me $450, being I just wanted to pay and leave I was forced to pay that amount. Are they allowed to alter their contract like this? I was pretty upset that I had to pay an extra $150 dollars.

-Separate from that case. I was in an accident with that car and was not at-fault at all. Got the drivers info and police report. Upon returning the car I had to pay a $500 deductible and they forced me to give them my personal liability insurance so they can file a claim. Why didn't they use the Enterprise one I paid for? They also wouldn't allow me to use my Visa claim either, stating it had to be insurance that dealt with Visa, not Enterprise, even though Visa states it will cover it. I paid for Enterprise insurance, why was it not used to do that?

-Will I get my $500 deductible back from the other person's insurance? Enterprise forced me to pay this even though it wasn't my fault at all. My total was over $900 just to rent a car for a WEEK.... it was the lowest tier of cars and this is outrageous......

The people at the desk would do nothing to help and were too lazy to call anyone. So I need help.