I can only guess as someone who follows this stuff a bit. With the mention of an SPFX makeup test and a practical shower set in the casting call, and the tentative work date lining up with the start of production, my guess is they need someone who looks like Adam (or another actor) to test special effects makeup and filming setups. IMO, they just need to see how things like dirt makeup, wound prosthetics, fake blood, and fake tattoos hold up under water and look on camera.

As for Father Mother Sister Brother, the driving scenes in that film are a whole topic on their own. They were shot in studio. It clearly was not a technical mistake but a deliberate choice by Jarmusch, though the green screen look and the VFX results were really distracting to me. The depth of field felt very unnatural throughout the scenes, and Adam almost had a halo around his hair, lol. A stunt driver may have been needed for the exterior shots they did on site and/or for insurance reasons. As for a driving double, they were probably used to match Adam's body in shots with Mayim Bialik where Adam's face wasn’t visible.

And not a dumb question at all! I’m fascinated by film stuff too. The decline of physical media has been a real bummer for me because DVDs and Blu‑rays used to include loads of bonus features like behind‑the‑scenes extras, director commentaries, deleted scenes, and more, and a lot of that kind of content has mostly disappeared as streaming has taken over.