For your first question:
I haven't tried it myself, but technically the method described on the page should work. On the other hand, as said on the page, it is still in early access, so there could be bugs or even security risks, so it is highly discouraged to use it in a production environment at the moment. But in my opinion that shouldn't stop you from testing it in a controlled environment. If it is currently in early access, it shouldn't take too long for a stable version to be released.
For your second question:
It isn't completely clear to me how you would use your login provider as ECM. But I can try to explain the authentication mechanism of ADF.
ADF actually doesn't have an authentication system of its own, but it uses the authentication system of the Alfresco back-end to login on Alfresco, so show up as logged in into ADF because you are logged in in Alfresco, not the other way around. Any authentication system that you would use, has to be compatible with Alfresco. This means that technically you could also log in by sending a simple rest command to the Alfresco service. Not all authentication related components would work well, but it works (I have for example written my own auth service to support Kerberos SSO).