Following Bauman's suggestion, I un-checked the “Check ID/IDREF” box in Oxygen > Preferences> > > RELAX NG. While this solves the immediate problem of the RNG schema being valid, it has the consequence of disabling the content completion/lists of values for attributes while editing the dependant TEI documents in Oxygen.conflicting ID-types for attribute "id" from namespace "http://www.w3.org/XML/1998/namespace" of element...
Are there other solutions, hacks, or work-arounds to solve this? The attribute content suggestion functionality Oxygen provides is crucial to our workflow...
Thanks in advance for any advice.