I would rather Ravenloft be a region in the Shadowfell than vice-versa. Ravenloft is its own unique world, much like Greyhawk or FR. And it's a big prison. Not one where anyone other than occasional PCs should be jaunting in and out.
Edit: Though Mearls is a heathen for using the term "ring" in reference to the Inner Planes, since (as Monte Cook tells us in The Inner Planes) they're more of a sphere.
I really do not understand why WOTC (and post Gary TSR) feel the need to have a very detailed default cosmology as core and published settings must conform. AFAIC, Each campaign setting should have its own,.and be detailed in the respective setting book.
But I'm a little bewildered that they didn't go with a more inclusive, modular idea: here's a BUNCH of planes for your game, weave them in and out as yous see fit, include or exclude or roll together planes that work for you or whatever.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.