Well, I heard back from S4C. Chwarae teg iddyn nhw, they sent a quick and detailed response to my questions. They can't make Pobol y Cwm available internationally due to rights restrictions.
I really wanted to see the second series of Lan a Lawr, but that won't be on either. I'm really disappointed, I loved that series. No plans for another series of Dim Ond Y Gwir, but there will be another series of Byw Celwydd (to be broadcast early next year).