But you cannot use the tickets at any intermediate station as I
understand matters.

This was discussed in another thread and although that is what the TfL
literature says I believe the conclusion was that the situation had now
changed though I must admit I didn't quite follow it.

Haven't looked at Tottenham Hale or Seven
Sisters but I'd imagine there would only be an interchange validator at
Seven Sisters as the street entrance is gated

Seven Sisters does indeed have validators on both sides of the passageway
leading to and from the WAGN platforms. They have been there several months
at least and were presumably installed at the same time as those on the
gates.