I just had another skim through this to remind myself…I did find some northern usages but I had to look quite a long way before they became apparent, so I don’t think it’s going to cause any particular difficulty to people who have started learning using the southern version of the courses/challenges.
It has other difficulties, however, and it’s not a book for beginner readers, in my humble opinion, even though it’s a children’s/teenager’s book, and even if you have the English version side by side with it.
However, when one is ready for it, another one to consider might be the “Cyfres o Ddigwyddiadau Anfoddus” series by Lemony Snicket, which I discovered last year in Wales. It contains what would be called southern usages but again, I had to look quite hard to find them, and I don’t think it would present any particular difficulty to people (like myself) who have come up through the Gog route.
These were originally written in English, and it says on the frontispiece “Addasiad gan Aled Islwyn” - Addasiad is “adaptation” rather than “translation”. This may mean that it is not a literal translation from the English. I imagine the English versions are not hard to find to compare with it, but I don’t have one.
Anyway, it seems to be supported by the Welsh Book Council and I think it’s from a reputable publisher, so I’m sure it’s good quality Welsh and not some google translation. (There are some of those out there as I’ve discovered lately, so one has to be a bit careful with translations, and ideally, it’s probably better to go for something that was written in Welsh to begin with, unless one can verify the bona fides of the translator, or at least check it with a native speaker or very experienced speaker).
On your point of “massive input”, I agree, and am doing what I can myself in that direction, both listening and reading. (But I did not start reading until well into Course 2 of the original SSiW).