"They were around, but were they known or buried in a manuscript somewhere?"

'The Twa Sisters' was in Anna Brown's repertoire, which gives us a 'sighting' in 1783. Given the subsequent popularity of the ballad, it would be remarkable if it had not been widespread in Scotland in the 18th century. Most of the North American variants, though, follow the 'Bow down' refrain pattern which I would associate more with England.

Re 'The Elfin Knight', does anyone (Steve?) know whether 'The Cambrick Shirt' appeared for the first time in the 1810 printing of 'Gammer Gurton's Garland' (as referenced by FJC), or the 1783 printing - as claimed in certain places on the web?

Bronson printed several 18th century tunes connected to 'The Elfin Knight' by the refrain 'My plaid awa', although he was cautious about making a definite connection bewteen these and Child 2.

I realise this is an obvious point but, since so few people were actually looking for ballads in the 18th century, it doesn't surprise to me that few were found in that period.