Well, since the game made no mention of Jenova getting aid in traveling across the galaxy, it's most likely to assume that she didn't need help.
Also, the Weapons were hardly new defences. They were there, I'd imagine, when Jenova first came.
I've asked this before: how could ANYTHING besides Holy and the Lifestream stop Meteor? Those are the only two that have that power.
How do you attack a Meteor? No plasma rays or anything would likely penetrate the atmosphere.
What point exactly are you trying to prove?
To your last statements: Meteor was the obvious threat. After that, Sephiroth. Jenova was the "behind the scenes" bad guy.
