As you say, we don't know who 'wrote' folk songs, which is a pretty fair indication of its working/peasant class origins - educated people tend to sign their creations. Where else would you find massive body of totally anonymous work.
As you also you say, we know so little about its history and origins, the reason being that the collectors (largely middle-class) tended to assume that the singers had nothing to offer apart from their songs.
It is almost certain that the sea songs were made by seamen who had experienced the conditions described - compare them to the creations of Dibden, Masefield et al. Similarly, the bothy songs present too accurate a picture of 19th century farming life in North East Scotland to have been created by outsiders.
The broadside presses certainly played a part in the distribution of songs, but even this is a two way street - existing folk songs being adapted and sold on broadsides - we know this from the fact that Travellers in Ireland were still participating in the trade right into the 1950s.
It has been a constant theme throughout the history of folk-song scholarship that the 'lower' classes did not create folk song, and were incapable of doing so - the only thin missing has been - a shred of evidence (apart from 'gut-instinct').
Let's see what happens with your question.
Jim Carroll
PS It's not a question of the middle class 'hi-jacking' anything, no more that it's a case of 'townies' taking of country-based music. It's simple a matter of people, whatever their origins, recognising the beauty and value of folk song and enjoying it - I hope!