1) I think it’s established Naminé is pretty incorruptible due to the very unique circumstances of her birth, plus the fact that she’s the Nobody of Kairi who has no Darkness in her heart. I imagine that, since the true purpose of the Organization is to make Xehanort vessels, she was most likely not considered because it just wouldn’t be possible.

2) I think he can use a Keyblade, but then I understand Nomura being tight-lipped about it would be because of spoilers. As for why he doesn’t use the Master’s Metblade - ???? I mean, theoretically, he could’ve lost that ability when he forced Terra out of his body and lost his memory, but then at the start of 3D we see him calling that Keyblade. I think it might be important to consider that one specific Keyblade can only really exist as a single entity - Roxas and Xion have to share Sora’s Keyblade as he sleeps up until the point Roxas grows his own heart (and then when Xion disappears, he can use two). Now considering the fact that during what we’ve currently experienced to KH, the Master’s No Name Keyblade is tied to Master Xehanort. When he became Terra Xehanort, it was Terra’s body and a weird mix of Xehanort and Terra’s heart (as far as we know), so maybe in the end Xemnas - being technically a Nobody to neither Terra or Xehanort - didn’t have access to it after all? Or maybe all the time travel resulted in someone else having the No Name all this time during the events leading up to three?

It is a mystery.