Before I purchase this game I'd like to know if the gog version has native xbox controller compatibility without using 3rd party software. If not then does the game play decently with the keyboard/mouse setup?

Also, without giving away spoilers does this game really end on a cliffhanger? In other words, is it not a complete game in terms of plot?

The game has no controller support, but plays (imo) fine with keyboard+mouse.

It's a complete story, but a scene after the credits sets up conditions for a sequel.