There are at least 2 launchers (GitHub - Fris0uman/CDDA-Game-Launcher: A Cataclysm: Dark Days Ahead launcher, GitHub - qrrk/Catapult: A cross-platform launcher for Cataclysm: DDA and BN), there is a soundtrack (GitHub - Fris0uman/CDDA-Soundpacks: A continuation of the CO.AG soundpack initiated by SkyBreach), game right’s simply cannot be affected whether game is on Steam or not (you can read more on Steam here - Steam Dark Days Ahead).
As for point 4 - you can read more on CDDA design in CDDA Design Document. It does not prevent creating mods that have different design or even lore.