I actually buy lightnings Gunblade, because it transforms between the two. It's never both at once.

And I can buy a young child being able to handle death, especially considering how early it happened, and how much. It's a bit of a stretch, but within the realms of my suspension of disbelief. (Not that I like Eiko, mind you).

And eh, I recall the Gunblade being a SeeD signature, though it's not really relevent one way or the other.