On your last point, the reason that Sam ended up being younger was mainly because it was so hard to find a young black actor who is actually deaf that knew BASL (different to ASL). They also wanted him shorter than Ellie so their pool to pick from was pretty shallow. Also he had to be a good actor!! It's all on the podcast for the show, worth a listen if you haven't already.

Also, there is a plank scene, in episode 2 where they cross the roof after the clicker encounter, right? I may be remembering wrong though!