Whether synchronization is checked or not the user will be added to DNN. What synchronization does is sync user roles (if the user is in a group in the AD and the same role is in DNN the user will automatically be added to it) and updates any AD user profile changes when the user logs in. If it is unchecked then it only happens the first time a user logs in.

From my understanding, ADAM does support ADSI.  You actually use a tool called ADSI Edit to manage ADAM.  Programmatically you use System.DirectoryServices.  I know ADAM also supports LDAP.  I'm not sure of the relationship between ADSI and LDAP in the stack (i.e. does ADSI use LDAP or are they peers).  Anyway... I "think" the provider you wrote "may" work with ADAM, but some things might need to be tweaked in the connection string.  I installed the provider on a DNN 4.6.2 instance and tried to point it to ADAM.  I get errors when trying to save settings under Admin > Authentication.  I've tried many different settings to try to "trick it" into connecting to ADAM, but without knowing what the source code looks like, I'm just wild guessing.  I'd be happy to try to help with ADAM support if I can get a look at the source code.  I've seen several questions regarding the availability of the source code.  Sorry to bug you with another one.  Any estimate on when that will be available?