[dekket]

Grimward,

I presume that the order will have fossilised a bit since arriving on Athera, as the Compact will have halted any further experimentation. There would have been no increase in the net powers of the Koriani order since they made landfall.
Thus new initiates could only learn what they could be taught by older, wiser, higher circles.
As for the original development of the Prime, it could be proposed that the Prime (or Grand Mistress/First Lady/President/whatever she would have been called back then) would have been the head of the order back before the Koriani discovered/learnt magic, out in the star-faring society of humanity, before it collapsed entirely. If the order was already set up with the multiple levels of initiation (similar to the Masons, perhaps) then it would be only natural that the head of the order would hoard the most secretes, including magic. The next level down would know slightly less, and so forth, all the way down to the novices knowing the bare minimum to be able to operate.

And I wonder if it is just the Prime's memories that are stored in the Waystone? Perhaps more than an Iyat is stored within?

I will have to pull out Zelazny again, as I can't quite remember how the Shadowwalk ability works.

Doesn't help that Shades in the Forgotten Realms novels have an ability that is called Shadowwalking, where they can jump from one shadow to any other (by passing through the elemental plane of Shadow).

Will re-read Amber series and ponder what is similar.