FOV is determined by the distance of the screens from your eyes, which are user-specific. Lots of users have reported [0] that switching to a 21W [1] light seal, or removing the light seal altogether [2] provides a massive FOV increase.
I'm pretty surprised that a product that's spent this long in development and is being released with such a high price tag would still need to be modified by users for what is arguable the most critical spec. And also I have no idea was 21W and 33W refer to so it seems unlikely the average user will have any idea.
I have no idea. I’d imagine using a light shield increases the relative brightness of the displays, which could be important to Apple.
Some users cannot use the thinnest lightshield: their eyes or eyelashes touch the lens.
Apple did make them interchangeable and exchangeable (for free), so it seems like the major FOV issue is with their scanning and auto-selection (for some users).