Congrats on the guitar, it's a looker!
As others have said, Private Stock and Artist tops are not marked with a designation, and never have been.
The decision to mark a piece of wood a 10 is a subjective thing done on the spot by employees at PRS. It's merely a marketing term.
Yes, keep the guitar cased when not in use. On the other hand, there's no need to be afraid to play it in a room with normal lighting. It's not a big deal, it's supposed to be played.
Northern Lights finish does tend to fade, and that's presumably why they discontinued it. I have a PS model with Northern Lights, and it's still very vibrant because it's always been cased when not in use. However, I've noticed that the purple-stain areas have begun to become less distinct in contrast to the lighter blue shades a little. That's probably happening because the wood absorbs some of the pigment.
My guitar is an early 2016.