As an update to this, I decided to order the Quicksilver van service from WDW to UOR and not rent a car. We will use DME between MCO / WDW.
For Quicksilver, I ordered one front-facing car seat and two booster seats, which is the maximum they allow you to pre-book. It was easy to process through their website (link below) and they called me right back to confirm the details. I’ll report how it goes when we’re there in May/June!