Well. After some searching, I have discovered that new PCs cannot be added to a game once it has begun. Which means if a PC dies, they're done. The party gets smaller. That's disappointing.

The GM mode functionality that DOES exist for this game is very good (some of the best I have seen), but it's just not quite there for me. I won't be asking my friends to buy copies of the game unless they want to co-op the main story with me or play whatever adventures other folks make with the editor. The game is still very good without GM mode.

Now if this functionality were to be added in the future by Larian or modded by someone, I am all in. Call me a GM. Until then, I'll just run my tabletop game at a table.