It is implied that Quiet Robe's will was so strong the X-parasite who infected him sacrificed itself willingly for Samus so she would fulfill the promise she made: to "end this once and for all"
Raven beak was never immune to X-parasites or had Metroid DNA for that matter. You see his fellow mawkin-tribe chozo infected by X-parasites during the game. And you fight some of them too. Because he cannot control them he replaced them with robot-chozo soldiers.
The suit change was a result of the metroid DNA fusing Samus's body with her suit, giving her a metroid-like appearance. Second, since metroids can absorb energy\aion from their victims, Samus could do the same. And with Kraid, nobody really knows, but I think the Makin may have possessed Kraid DNA at one time or this is a seperate member of the Kraid species.
