My $0.02 worth....
The fact that they sent you three different contracts does not "proves this is a fraudulent claim". What it does prove is that someone in their office screwed up and sent the wrong contracts and estimate.
Also, having rented vehicles most of my life, I am aware that you need to read your contract. There will probably be a provision in it saying that you are responsible for any damage to the vehicle until checked in by them. That means if you leave it in their parking lot overnight and someone hits it and flees, you could be found liable for the damage.
I am sure that you inspected the car before you drove it off the lot and would have noted if the scratch was there, right?? So, assuming the scratch wasn't there, you need to get a copy of the rental return report that they filled out after your return. If there is a scratch noted on it, you are very probably liable for it, unless you can show that it happened AFTER the rental company had already 'received it' back into their custody (highly unlikely to prove).
In my opinion, the only challenge you have is to the amounts being charged. $864.00 for a scratch seems excessive. Read your contract and see if you are also liable for the 'missed rental days' that they car was in the shop. That is the only way that I can see the amount being charged.