Interesting. You couldn't find a 7 seater. When I was looking around the dealerships, and checking inventory online, they were everywhere.
But then again, it is still 2016, and you were looking for a used 2016. That usually means that availability would be limited to whatever the rental car companies are trading in, because most private owners don't trade in their cars within the same model year. I hope you saved enough money for it to be worth not buying the 7 seater that you really wanted.
I imagine that you would be able to install the 2nd row 3 seat bench. The hardest part may be pricing and availability. Since seating is rarely a collision repair item, it's not as easy as getting a bumper or a door. You will probably need all the hardware, brackets, and latches. You may need to drill a hole here or there. If you're not worried about functionality, and only seating, then you could probably get a torch, cut what you need to, and weld it in permanently. I added 3rd row seats to an older SUV that didn't come with 3rd row seats. I've also added flip and fold jump-seats into the bed of pickup trucks. Once, I stripped seats out of an old school bus, and installed them into a cargo van. Being a guy who has owned Jeeps & trucks most of my life, welding and drilling holes into cars does not bother me. But other guys would never take a Sawzall, drill, or torch to their cars....even if the end result was getting what they wanted.