What a sweet, gentle story! I love the concept of the room with doors offering "what-if's," something Severus would be familiar with as I think a big part of him lived a life of constantly considering the "what-if's," so it makes sense that it would be where he finds himself in the great hereafter.
It was wonderful to see him learn that things could not truly be different without changing everything, including things he would not have changed. And it was beyond wonderful for him to see what could be and cleave to it, insist on it and hope for it, and finally accept it once he awoke. Beautiful! Brava!