The latest update (
Zandalor's perspective on recent events, SP video, reward updates, more undies) clarifies the tiers.
Yes, for the physical version tiers, the DVD and digital versions are separate, so you can keep the DVD and give the digital version to someone else.
You will have a choice where to get the digital version (at least Larian Vault, Steam or GOG).
Online co-op is through Steamworks. The GOG version of the game would need to use the LAN mode to connect online (I don't know if the game supports that directly, but there are programs that can set up a virtual LAN for gaming).
