John
As others will tell you I am at the extreme end of scepticism when it comes to the early collectors. At one end are those that believe all of the ballads came from oral tradition and at the other, like me, there are those that believe that there was a great amount of rewriting going on and mixing and matching. The truth must lie somewhere in between. The thing is no-one can prove it either way. I have been studying closely the Child ballads, broadsides and oral tradition for more than 40 years now; on the other hand there are academics with at least the same amount of study and who are more highly qualified who take the opposite stance to me. All I can say is that most collections exist in manuscript form and give details of the informants. With these we have really to take the collector's word, people like Ritson and Motherwell, although Motherwell admitted to some adulteration in his early publications. The likes of Buchan and Scott we just cannot trust(IMHO)

As for American versions, as I said before, the British ballads must have come across in the heads of those migrants originally and the majority must be genuine. There is no reason why even the earliest settlers wouldn't have brought their ballads with them. I would just avoid Niles and Reed-Smith, but this is only MY opinion. There are also those that are so close to the British versions that they must have come from books, possibly even Child itself.