Here's the thing about the original Rising that Kojima Productions was working on (if they actually were seriously working on the game), aside from the fact that I can't recall them ever saying it would be a stealth game (I'm fairly certain Kojima was saying from the beginning that it was going to be an action title, and frankly, doing anything else with cyborg Raiden after MGS4 would have been silly), I don't think Kojima Productions could have pulled it off. And I'm willing to bet the entire reason they're just producing it now and Platinum have come in to do the actual heavy lifting is because they either knew they couldn't do it or didn't want to have to do it. And frankly, the trailer they showed last year didn't look a whole lot different from what they showed now. The only real differences I saw being that this one seemed to be faster paced, and looked like it controlled better than what they were showing in 2010. Neither of which is a bad thing if you ask me.

As to why Raiden is back to being a cyborg mercenary nirojan, I would assume it's out of necessity to fight the enemies in the game. Technically, he never stopped being a cyborg after MGS4. He just got a more human looking body.