Hameed,
1. I am not sure exactly what you are referring to here. Only DocuWare can login to DocuWare. Of course, a 3rd party app can leverage the DocuWare API to perform a login.
2. Yes:
http://[your server]/DocuWare/Platform/
Contains the documentation for the API. The API can be obtained from Nuget.
3. Yes
4. Yes
5. Yes
Phil Robson
Senior Director Support Americas