There are some scummy dealerships all over and it's not overly surprising that these guys have decided to stonewall (bearing in mind that we're only hearing one side of the story here).
Regardless, it seems like a straightforward problem to me.
- Ask them to take responsibility
- If they fail to do so, file an insurance claim
- Let the insurance company worry about recovering from the dealer's insurance company
- Go back and sue the dealer (small claims court or otherwise) for the deductible or loss for replacementand other incidentals.
I'd also be bombarding Nissan with complaint letters as well as the state's Attorney General's office, and of course there's always social media. Those things would come after getter the car repaired, replacement rental, etc.