I think the assassination plot was from one of the manga. Not sure if they will use the whole thing or just a failed attempt on Dessler's life that is caught rather than the manga version (Dessler's armed harem to the rescue?)
Could lead to a civil war in the Empire at the time Yamato is closing in, weakening the defenses so that they don't have to blast their way through tens of thousands of warships shooting at them. Cause even Yamato's shields and armor have limits.
But then Yamato has its own problems.
About the only sure thing in this series? That Yamato get home with the device to clean up Earth. (or at least the device makes it to Earth). But people living and dying? The game is different. No one is for sure safe anymore.
My assumption would be that the end of this season will at least have everyone around who was suppose to be alive for the original second season. But that is not a certain thing....even if they have introduced many characters from later seasons and films already.