Impossible to answer, some will argue it's not, other will argue it is.
In my own professional opinion a good MVVM/application framework like Prism can save you a lot of time and effort and boiler plate code, the bigger the app the more these frameworks make sense and start to pay dividends in reducing development time.
I used MVVM Light 1st which is a great little framework but for more complex apps I can certainly recommend Prism.
Your namespace can be whatever you want. I just used Foobar as a generic non descriptive namespace for sample code, it's just the namespace of the PCL for your UI layer (Pages, converters, extensions etc). I keep my viewmodels/business logic in a separate PCL but not everyone does.
You could use DinePerfect.Extensions for example for your markup extensions or DinePerfect.Converters for ValueConverters. Just means if someone else picks up your work it's easy to find out where stuff is and what it's purpose is.
Well, the next Microsoft Build conference is in May, so i guess we will see the next stable release then.
The stuff that arrives for build is usually unstable as lots of features as pushed out at the last minute. I'd wait for 2 patches after release before upgrading a production app.
I'd settle for a previewer that works, Live preview has limitations of device and dev IDE been on separate networks, the Forms previewer that ships is useless, it can't handle apps that use constructor injection, only basic scenarios work and it seems to have been abandoned. A design tool would be nice, I think the 3rd party control providers will provide something, I noticed Telerik has moved it's theming capabilities on a lot but that shouldn't have to be the case.
In my new role we're using the vanilla listview, I'd forgotten how bad it was (used RadList before), iOS 11 brings a bunch of issues, device rotation causes all sorts of weirdness and in UWP it's not good, we'll switch to Radlist in this app.
My suspicion is you won't see any fancy nice to have features in the vanilla listview as that's part of the marketspace for 3rd party control providers, it's like the vanilla datagrid in WPF, it was awful but that left plenty of space for component vendors to fill the functionality gap.
I'd settle for core framework stability like @ClintStLaurent, oh and better stability of VS2017, the main thread hangs so often it's not funny and this is on an i7 with an SSD and 16GB ram.