Bob deinterlacing is used because it’s fast but it doesn’t look particularly great in my experience. Even compared to a CRT. Considering you have a PAL GameCube, I’d recommend just getting a Wii with component cables to play games in 480p and keeping the pal gamecube for use with the gameboy player in RGB. Component cables for GameCube are expensive regardless of whether you get the official ones or not. And pal gamecubes normally don’t allow 480p either. To answer the second part, connecting audio via the composite cable works just fine. And you actually have to do it that way for the official component cables.