Larian Studios Forums Divinity - Original Sin Divinity Original Sin - Mods {WiP} Tinker, Tailor, Hunter - a Harder mod

I don't think you actually need to kill Baccas Rex to progress the story. You might need to kill him to beat the game, I forget if he drops a starstone or not, but basically, you can choose to not kill him and just go to the next zone if you want to, nothing should be stopping you from that, and I don't think it would stop you from investigating the Immaculates. You could probably do any of the other main starstone quests and then go back to Rex.

The game I think is only linear in that it forces you to collect every single starstone.