Hi Danest,
I have noticed that reading the faction heir's, at the start of the game's, biography explain quite alot about the collapse of the Seleucid empire under his rule. Also, as you play through as any faction, you receive messages as traits in your faction leader which contains lots of great historical information about events that were happening around that time, including about the great Seleucid Empire's collapse. Good luck with finding out your history, cheers!