You are correct in suspecting missing "standard form" metadata.
The first two of you methods, "dbms_aw.aw_create" and "dbms_aw.execute" will both create an empty AW with no standard form metadata. Such an AW is fully supported and can be accessed through OLAP DML, but it is not visible in AWM and does not show up in the various USER_CUBE_* dictionary views.
The third method, dbms_cube.import_xml, should have created a valid "standard form" AW that was visible in AWM.
The error you got from your final method is easier to explain.
The problem is that the import_xml procedure can create a new "standard form" AW or modify an existing "standard form" AW. But it will not convert an existing non-"standard form" AW into a "standard form" AW. I suspect the same thing happened in your third method, but no error was reported.
The solution is to delete the empty AW by hand and then run your third method again.