like the songs of MacColl and Leonard Cohen
A further complication
One draws its inspiration from traditional song - one does not.

I don't think there's much difference. Cohen used an idiom based on French chanson (which has a folk underpinning) and apart from a few gestures towards rock&roll, stuck to it. MacColl ranged more widely, from "Shoals of Herring", which is faux-Hebridean, to "The First Time Ever", which is somewhere in between Jerome Kern and Steven Sondheim (I don't like it and I don't much like Sondheim either). As a music-theatre composer he had to be a stylistic magpie.

At any rate, the singer-songwriter genre is not defined by stylistic features, but by social ones - who performs it, where and when, under whose auspices, and who for.

(And thanks for starting to make an effort with the whitespace).