Re-enabling the culling of portal polygons during their Clipping phase paid off - I am now detecting five portal quads are being eliminated during the search for 'true portals', and their geometry matches that of the inner box faces - I have eliminated my 'fake portals' correctly.
The sixth and final leaf now contains 4 portal references, none of which lead into the interior of the inner box

I can see one remaining problem: two of my portals contain 5 polygons, where there should be 4.
Furthermore, there appears to be a pair of quads with the same geometry (actually they differ by 0.00000002).
I need to look into that.