"why would print have helped the development of ballads among the overwhelmingly illiterate lower classes?"
....and the totally illiterate Travelling people, to whom we owe an enormous debt for salvaging and keeping alive many of our ballads.
Literacy played little part in preserving our ballad repertoire in the tradition - there is a great deal of difference between being able to read and using the skill - we found that James Hogg's mother's attitude to writing down the songs persisted with many traditional singers (those who could read) and published texts were treated either with suspicion or with reverence, in which case, they were treated as sacrosanct and remained unchanged.
This was brought home with a storyteller we recorded who gave us a version of the epic tale 'The Pursuit of The Gilla Dacker'. We found a published version in P W Joyce's 'Old Celtic Romances' and asked the storyteller if he knew of it.
"Yes", he said, "but he's got it all wrong; those fellers never got it right".   
If your reference to some published material being "ludicrous", is aimed at Buchan, once again, I find your somewhat definitive language more than a little disturbing. The 180 year-old Peter Buchan 'controversy' remains unresolved after all that time - to suggest otherwise is misleading.
As Brian said, "Buchan has his defenders", including D K Wilgus in his 'Anglo American Folksong Scholarship' who points out that if Buchan was 'improving' his texts (a totally unproven accusation) he was doing nothing different than any of the other 19th century ballad anthologists.
Jim Carroll