Was just thinking, a possible point in favour of your theory that the original Celts were SW Euro-like was the Italian affinity that NW Euro countries scored surprisingly high amounts of in the Viking paper.
https://i.postimg.cc/9fv9JwGy/Got-VW...-estimates.png
English = 19% 'Celtic' in your model, 18% (17-20%) 'Italian' in the paper
Irish = 11% 'Celtic' in your model, 9% (7-12%) 'Italian' in the paper
Scandinavians get 5-10% of it as well, which would be hard to explain through Romans or something.
Might be a coincidence, but just putting it out there.