I've exhausted my Google powers looking for information on the transition from Pagan to Christianity in Wales... Most things I've found are along the lines of "By the withdrawal of the Roman administration in the early fifth century it is likely that Christianity had become the dominant religion throughout the province." Are there any solid accounts of the interaction between Pagans and Christians in this period?
"Are there any solid accounts of the interaction between Pagans and Christians in this period?"
No. There are very few written sources for late Roman Britain and none discuss interaction between pagans and Christians. Little is known about the 4th century British church, except that it existed. 4 British bishops attended Constantine's church synod at Arles in 314.
Judging by the popularity of St Martin of Tours, a controversial 4th century reformer, the British church was fairly populist and plebian. Bede mentions a church dedicated to Martin in Canterbury, built during the Roman period. Victricius, bishop of Rouen, visited Britain in around 396, and was a known admirer of Martin. Martin's biography emphasised his lower class dress and habits, and paints him as a champion of the poor and weak against the government and rich bishops who submitted to the powerful. Victricius was invited to intercede in a dispute in Britain. That would indicate a populist party in the British church. Further evidence of it's egalitarian nature comes from this guy...http://en.wikipedia.org/wiki/Sicilian_Briton
I hope that helps a little.
Your best bet is the excellent T.M. Charles-Edwards, Wales and the Britons 350-1064, Oxford, 2013. Maybe refer to (eds) N. Edwards and A. Lane, The early church in Wales and the west : recent work in early Christian archaeology, history and place-names, Oxford, 1994.
One chief problem of pagan Christian interaction is that we usually only have one side of the story (the Christian one). These ecclesiastical sources saw little issue with painting the pagans in whatever manner they saw fit. At any rate the deep and mystical past of early medieval Wales is beyond my remit but should you wish to know anything about the topic Charles-Edwards' book will lead you to it (either directly or through his bibliography).