I'm guessing there will be more specific answers to your questions from the product team at some point, maybe in the form of a blog post.
I can tell you with the limited testing that I've done, that policies will work the same way with Defender as they do with EP, and that you can tell policy is applied to Defender by opening the "About" dialog and seeing it listed there, just as
you can in EP. Also, if you go into the Settings dialog (which is now in the new Windows 10 style) you'll see that the settings controlled by policy are grayed out and unchangeable (unless your policy specifies that settings can be configured by the user.)
You can also look in the registry under HKLM\SOFTWARE\Policies\Microsoft\Microsoft Antimalware.
The "Scan with" right-click menu option seems to have been removed. The nearest alternative seems to be using the custom "scan now" option in the Windows Defender GUI to scan a specific folder. You can also scan specific files via the
MpCmdRun.exe command line interface located in C:\Program Files\Windows Defender. You might even be able to create your own custom right-click menu option that invokes MpCmdRun.exe to scan specific items.