Just an idea that came to me almost out of nowhere - you know how every listening practice after the first is a dialogue? I find myself wondering if it might be worth having a mixed listening session for each; wherein, let’s say, Aran and Iestyn speak their respective dialects to each other (with the pair swapping roles between performances). I can see two fairly obvious benefits from this - in the long term, it gets people used to hearing more Northern/Southern ways of saying things from an early stage, while in the short term it means that at least some listening material for Southern Welsh will be easier to produce while Cat continues to recover.
Just a thought; feel free to use it or not as appropriate.