It's been confusing for me to figure out why win10 S users will have an optional feature to upgrade to W10 PRO instead of Other edition(Home, Enterprise etc.)
My guess would be that this is being offered to entice people to try Windows S without fear knowing they can always "upgrade" to "full windows" if they want. That would be my thought.
Yes. Get them first 'safer and faster' and if they need that/those app(s) that aren't yet available through the store (which I will doubt more and more since it's incredibly easy to add a win32 app to the store, example I did here: Cockos Incorporated Forums - View Single Post - Reaper in Windows Store via Windows Desktop Bridge) there is always the option to enable all of 10 Pro's features, which include 'non trusted' win32 apps.
No Windows 10 S is Windows 10 Pro based. The free upgrade is a promotion until the end of 2017. Enterprise is a different animal. Enterprise is a licence for multiple computers and doesn't make sense to install of machines that aren't in an enterprise environment.