Speaking personally, I've had issues with having no internet for days or even weeks on end on occasion, and have had no issues using Steam or accessing any of my games during that time. I'm not discounting that others have experienced problems, but from my perspective, it works just fine. And you don't even have to run Steam in offline mode to use it offline, providing it's already running and had logged in when internet access was possible.
If Steam decides it needs to update on next start and you then start it without an internet connection you're in trouble. Now this was quite a few years ago but it was horrendously annoying.
Yes, if you get D:OS on the Larian Vault it comes with a Steam key.
OK, I guess this also goes for any extra copies I might have added as addons? (I guess they can activate them on the Vault and then retrieve a Steam key if they so please?)
The mail was referring to the digital version of the game. All kickstarter tiers with a physical version also have a digital copy.
The game is DRM free, so if you can not connect for awhile it would just mean you could not get new updates. You would still be able to play the game.
Maybe I should just pick Steam, it would certainly make figuring all this out easier :P