"ey21 demo": I don't have a clue - maybe someone else does.
I don't think we have the process written up in any one place.
There is a rather long thread here about what one person did to make their "Demo Kindle" useful without actually un-demo'ing it.
There is a lot of things written up on this site (you might notice that this post id number is greater than the 2.8 millionth post).
The crib sheet on finding things:
https://www.mobileread.com/forums/sho...d.php?t=237083
I haven't actually done the un-demo'ing of a paperwhite myself, only read and commented on other people's efforts.
Other members here can probably be more help than I can at this point.