Will re-post after their answer.
It refers to Grial UIKit updates itself, not Xamarin.Forms, it's clearly stated that they support Xamarin.Forms updates. I think the current version will work with no problem with all XF 4.x versions (if not, Grial will very likely fix it)
Don't see why it shouldn't work on tablets, it's just custom Xaml, themes and some controls after all.
As I said in 4, it's built on Xamarin.Forms, it's just a bunch of (very good looking!) Xaml and themes, so it will run on every mobile device Xamarin.Forms work with (except for custom controls that need a Renderer, I think they don't/won't support UWP as Microsoft and Xamarin team themselves don't care about it anymore and is dying...)
Better ask them, just email or DM them on Twitter with all your questions, they will reply (they're very professional guys).
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