All right, here goes! Though I should note first, I'm definitely a fan of reader interpretation and don't think my intentions as the writer are the be-all end-all. But since you asked:
I think it would be perfectly valid to assume that Sam is already possessed by Lucifer or the YED or is soulless here, but to me, this is 100% Sam. He's already explicitly rejected John as his father by the end of "Dream Deferred", and has gone from being an extremely damaged, unhappy hunter's kid to feeling like he's found his place in the world following the YED's plan. Accordingly, I don't think this happier, embracing-his-evil-destiny Sam would feel the same need for self-destruction that he did when he was younger and still 'trapped', hence the less fraught behavior around Dean. I don't think this Sam - who gave up his dreams mostly so he wouldn't have to live without Dean - would ever let Michael lay a finger on Dean. So I think this Sam would be quite eager to secure Dean's allegiance to Sam, and to keep Dean for himself.
Aaaaaaaaand now back to your regularly scheduled programming. :D
Re: 2/2
I think it would be perfectly valid to assume that Sam is already possessed by Lucifer or the YED or is soulless here, but to me, this is 100% Sam. He's already explicitly rejected John as his father by the end of "Dream Deferred", and has gone from being an extremely damaged, unhappy hunter's kid to feeling like he's found his place in the world following the YED's plan. Accordingly, I don't think this happier, embracing-his-evil-destiny Sam would feel the same need for self-destruction that he did when he was younger and still 'trapped', hence the less fraught behavior around Dean. I don't think this Sam - who gave up his dreams mostly so he wouldn't have to live without Dean - would ever let Michael lay a finger on Dean. So I think this Sam would be quite eager to secure Dean's allegiance to Sam, and to keep Dean for himself.
Aaaaaaaaand now back to your regularly scheduled programming. :D