Okay, having gone all through all the Northern material. What I really need to do is improve my listening speed and build vocabulary.
To help with my listening, I need to learn the Southern forms/ vocab. So it’s lovely to finally hear what @Iestyn and @Cat sound like. However, having started this, do I learn the Southern forms for speaking, or just for listening. If anyone else has done this your help would be appreciated.
The Northern Welsh for the sentences flows out of my mouth quite naturally now and it seems wrong to start telling my brain it’s wrong after having put the effort in. So is it okay to use the Northern forms and just listen to the Southern words back? I know it does not really matter, I’m sure it’s possible to choose which word to use, once something approaching mastery is achieved.
Also is ‘Cymraeg’ really pronounced differently; is the shift more than simply saying it with a different accent
That’s approximately what I have done when dabbling in the northern lessons. Quite a lot of it is fairly interchangeable anyway, so it’s nice to get some more options in the repertoire…
Yes, absolutely. That’s what I would expect to give you the best mileage… 
I think that’s what it’s like in real life, anyway:
“Oes llaeth 'da ti?”
“Nagoes, ond mae gen i llefrith soya.”
We all use the words that are most comfortable to us, and we all understand each other 99.9% of the time 
(in English as well as in Welsh).
I am an object lesson in that! I have said elsewhere that my chequered career, well, long and messy life, has meant that my Cymraeg is about 3/5 south and 2/5 north. Sometimes it seems the other way around!! But using SSiW as a way of re-learning, I sit and hear Iestyn say, “Mah ddrwg gyda fi” and I say, “Mae ddrwg gei i”, as an example! It happens all the time. Sometimes I switch to @aran because I think I must know some of this material and it transpire that I do, in Gog!! 
Already mentioned, but I think it’s unfamiliar accents that can be the tricky thing, together with people speaking much faster than one is used to. Or in rapid “bursts” which they seem to specialise in on “Rownd a Rownd”. Could be an Ynys Môn thing I suppose. I’m used to their accents after several years of watching the programme, but those rapid bursts still defeat me quite often. (“Meical” - I’m looking at you…).
This link (spawned by the unintelligable Jamaica Inn on BBC) shows clips to English language TV dramas and programs and you have to pick which answer you think is the one they are saying:
I haven’t done the quiz yet, but I assume they have picked some crackers and just shows that we quite often don’t undertsand large bits of what we hear on TV even in our dominant language.