No problem! BTW both the Ultra and Prime series actually use the same mounting bracket as far as I can tell. There's a separate insert that slides into the bracket to anchor the speaker and keep it from sliding out of the keyhole slots when mounted in either location.
https://cdn.shopify.com/s/files/1/0921/3560/files/Prime_Elevation_Ceiling_Block.pdf?v=1594238287
I've got three sets of Prime Elevations mounted high on the wall in my setup, but they all came with those inserts packed into the box.
EDIT: apparently the Ultra version uses a magnetic ceiling block, versus the rigid rubber one that the Prime Elevation uses, but the end result should be the same with either one.
You should be able to sign in to just the store app by clicking a tiny little box on the subsequent window that pops up during the sign-in process. That's how I sign in to the store for app downloads without having to use those same credentials to sign in to the PC.
That box is intentionally made small and easy to miss because they obviously want you to use a MS account across the whole machine, but it's there if you move slowly and look for it.
That said, you shouldn't have to sign in to download any app updates from the store. You can also download new apps without signing in-- the license will just be tied to the machine instead of your personal account if you do that.
This unhelpful error is just MS being MS. Best option is usually just to leave it be and check back later, as it oftentimes really is a problem on their end.