Can I update Automation via the Standalone Launcher, or only via Steam?
Only via Steam for now, the standalone update will be released about a week later.
Do we get to convert our standalone keys to Steam keys today or sometime later?
Today right before or at launch! 
And I can use the standalone launcher and the steam version both?
If you have two keys and convert one, then yes. Otherwise: no.
And, for example, if my Steam account is banned or stolen, can I recover the key or similar?
No, that is not our problem at that point, as you probably understand.
Yep. Is true.
Will those who preordered still get the v16 and art book?
Yes, of course! As you see in your User Control Panel there is a note that you will receive V16s and the artbook. The V16s will be DLC on Steam, so you will get a free key for that if you bought the premium edition back then. The artbook will be a download offered on the User Control Panel page once it is done
Cheers!
I found a possible bug:
When you select the rear engine (not central engine) icon, and select one of the available bodies, the engine are central and not rear.
In the new hatchback body, it’s impossible to fit a tailight decently without the “cutting body” bug (like when you put a grille with a overlapped UVMesh).