Brian, if I'm not mistaken, the question about the second grate comes from the assumption you'd smoke with a Hunsaker ... but this is really for grilling in a WSM/drum instead, yeah?
The 18.5 remains curious to me. If they're using that platform for the Vortex it would mean they either have multiple models for WSM (nope) or that the Vortex is now a smaller diameter from what it was originally. So if it got taller, to retain capacity, that would also hint at why you wouldn't want to use a WSM's second grate with it.