I don’t think the “will secure their return” sounds that great. I would also keep “fallen Empire” (why ? Because it’s cool, it’s kinda “the fall of the roman empire”, I just like the way it sounds). Also, capitalize NEW REPUBLIC since FIRST ORDER is capitalized (and I assume the remain of the NEW REPUBLIC is going to be quite important in VIII-TLF).

Luke Skywalker has vanished. In his absence, the sinister FIRST ORDER has risen from the ashes of the fallen Empire.

Failing to convince the NEW REPUBLIC of this threat, General Leia Organa gathered allies for a brave RESISTANCE while the First Order secretly completes a dreadful new weapon.

No suggestion for the third paragraph.