1. FCKEditor  will not continue to be default editor in future versions. The provider team is working on integrating succeeding CKEditor 3 as new default editor. DNN PE however might include RAD Editor provider.
2. DNN will remain open source and will be able to be compiled (however controls included in web site doesn't require explicit compilation).
3. DNN will continue including controls, some will be based on Telerik (staring past version 5.2.0), those may be used for free. For other licensing details please stay tuned for the official announcement.
4. dito

My understanding is that the default DNN Text Editor will be the Telerik editor (not sure about the timing, exactly which version, it might not be until 5.3) in both the Community and Professional editions

I also believe that with the DNN Community edition, the Telerik controls assembly will be a custom one provided by Telerik which will only be usuable within a DotNetNuke web site.  Since they are providing the controls for the Community edition for free, they obviously don't want you to use those controls in some other web site or application.  They will be the fully functional controls, they will just check to make sure they are running in a DNN site.

With the Professional Edition you receive a developer's license and will receive the commercially available Telerik assembly, and can use those controls as if you had purchased a license directly from Telerik.

I'm sure more information will be published shortly on the Telerik terms of use