Well, for what i know, Interea is just a explanation of some events during Adastra. While i like to think that Interea takes place 8 years later (presumably when they get back together), Khemia takes place after Interea (since in Interea, Marco still hasn't started his mission). I just hope theres a Real sequel after Adastra/Interea, where Marco and Amicus get to be together again.