Are dwym and boeth interchangable?
I don't think so but I personally don't use "Twym". For me "Poeth" is "hot" and "Twym/Cynnes" is "Warm".
Yn y Wladfa?
No "Y Wladfa" is used for a specific area of Argentina not the country as a whole. Assuming that you were asking if "Yn y Wladfa" is an acceptable alternative.
No, I wasn't. I was said it because I'm wondering if they included Argentina for the people of Y Wladfa. Hence why I said "in Y Wladfa".
Is "in the Argentina" considered too archaic now, or also an acceptable translation?
I've never come across "In the Argentina" used in English, maybe "In the Argentine" but even then that sounds very old fashioned.
I've never heard it (UK)