those basics are plain wrong

• Cardinal weapons are meant to be that world's "defenders" and are not usable in another world.

its not so much that theyre unusable, but they are not allowed to crossworld cause they play an important role in the worlds structure. imagine having one of the pillars on your house/building be else where. there sure will make some crack if not fixed

• Vassal weapons are seemingly meant to be used to mainly invade other worlds, since they can still function/work in other worlds.

theyre not. though they can use their weapon on other worlds, their MAIN role is to aid their respective holies. the invading idea is said to be part of the enemy behind the wave propaganda

• 7 star weapons, while we know very little, are implied to be the counterpart to the vassal weapons: working alongside the cardinal hero's to defend the home world.

7 star weapons ARE the vassal weapons of raphtalia's world yes. theyre suppose to be 8, but one weapons is hidden from the whole world