@Surfer1 I've edited your post to remove the registration number, but left in the year of first registration identifier.
As Homer67 said, the argument here is whether the vehicle was sold as a 2018 model or as a 2018 registered vehicle. It looks as if it was sold as 2018 first registration, not as a 2018 model, so I doubt you have any comeback on that. Let's see what the experts think.
When you say you were "given the impression that the car had a Jeep brand warranty and not a third party warranty" how were you given that impression? The ad to me gives no indication one way or another. Did they specifically say so? If not how was it communicated?
What was their explanation (if any) for saying 1 previous owner when there were two? How long after purchase did you ask them about this?