I think this will work. This single quotes might not be needed, depending on the GUID format (which can either be ID_<number> or just <number>):
"//" + objectName + "[id" + "='" + objectID + "']"
If you retrieve by a single Id it will always be a single object.
You can use Core.retrieveId(context, id) for that.
The list of attributes is called Members.
BTW The best way to find this is Eclipse.
Can one of the AppStore solutions help you, either with the functionality you want to build, or to give you inspiration how to collect object information? Mx Model reflexion or Object Browser