"The imposition of styles and idioms foreign to a particular form results in that form being transformed. It becomes something different. Not necessarily something worse or better, just different".

Again, I find much to question in your quotation, Jim. For a start, it asks us to assume that the "particular form" is something fixed and immutable and incapable of change. This is quite simply not the case. I would argue that within the range of traditional singers there is more stylistic variety than is heard in folk revival singers, and what your "group of Travellers" regarded as 'straight style' might not be regarded as such by another such group because there is no such style that could be described as unitary. Not only are there wide geographic differences in approach, as far as we know, there are also changes over time. No one sings at the pace and style of Joseph Taylor and the other Brigg singers today. I am old enough and lucky enough to have heard five generations of The Copper Family singing their songs regularly over 50 years. The words have remained the same and so have the tunes with only slight variations, but you wouldn't say that the style and approach to the songs are the same by John Copper's grandparents as they are by his grandchildren. Actually, I think they all just go ahead and sing the songs in the way that feels right to them.
I take it that you are familiar with the recordings of Betsy Miller. She was quite elderly when she was recorded, but you can hear a sprightly approach to her singing. She is a good example of a Scots singer but I would hesitate to call her typical because I am not sure that there is such a thing. Now compare her singing with that of her son's..... big differences. You would almost imagine from the commanding drama of his approach that he brings to some of his big ballads that he had worked in theatre. Oh he did, did he? Then that approach was a natural development because we are all open to what life's experiences throws at us in what we contribute to the flow of the river of traditional culture.
So who was the best example of ""the clean, traditional sound" - Betsy or Jimmy?
Possibly both.
Possibly neither.
Probably there is no such thing.