Kingdom Hearts II is roxas a keyblade master?

Technically, Riku didn't 'steal' the Keyblade from Sora. It was intended for Riku. What would have ruined the whole story was if Riku had noticed back when he took it at Traverse Town and passed it back to Sora, that it was weird it ended up in his hand and he just willingly let it go. I do think Roxas is a Keyblade wielder but he definitely threw the wrong weapon at Riku, since Riku has connection to Oblivion, and Roxas meant to throw it, not be nice and pass it to him for a fair fight. I do agree with your observation that in the end, Riku may have the strongest heart, or at least willpower, in the game, especially after what he had gone through. I guess that does go in another thread though.

I'm not even sure that Roxas is simply a Keyblader because of Sora alone, so anymore now, any doubts I may have had, I cast aside because Ven and he are just too much alike to discount that as well.

 

If Sora is a keyblade master, the the other half of him is... Of course Roxas is a keyblade master...

 

No, Roxas isn't a Keyblade Master, and neither is Sora. Keyblade wielder, yes. Keyblade Master, no. As far as we know, the only Keyblade Master was Xehanort.