Dealership Problem

I would ask the customer support for a copy of the case file they started so you can have it for your records if you haven't already.
This way you know that they actually started one. Keep all oil change and maintenance record from before and after the
incident.
Plus documentation in writing that they told you it would be covered under the warranty. This way you will have your own file.
Oh! If you have any more correspondence with them write them a letter instead of a phone call. It holds more weight legally then a phone call. As it would be consider hear say unless it is being taped which you have to tell them you are doing.

This is my opinion.
I believe that is what you asked for and not name calling and arguing.

Good luck and hope it works out for you and your step daughter.