Amazing work! I'm glad you got it figured out. Feel free to distribute your download/install script.
As for why the Simulator doesn't respect the $PLAYDATE_SDK_PATH value; the Simulator itself is inside that path and normally shouldn't be pointed at a different SDK. As I noted before, it assumes it will use the same SDK from which it was launched from (which usually isn't a problem) so there isn't a need to use the env value since it can resolve the path automatically on launch.
Again, great work! I hope this is helpful for other NixOS users out there.