Apologies for the wrong figure reference, typed without looking, should have said the map figure 3, both diagrams esp. fig.14 (the second one you posted) appear to show the feeder once over the SNowdon ridge going roughly NW towards Bettws Garmon, whereas the map correctly shows it going roughly SW towards Rhydd Du and the Drws y Coed pass, before taking the RHS of that valley to curve round behind Nantlle village and approach Pen Yr Orsedd from the East and North.
I hope that makes sense!
Edit2: Just re-read your post - I had not appreciated you hade backwards engineered the firts sketch. All makes sense now, but my comment holds.
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Grahami
The map is the territory - especially in chain scale.