"Like the "finger in the ear" can (I suppose) be attributed to MacColl, can the speaking of the last line of a ballad be attributed to one singer?"
Never heard MacColl do it - it's an Irish thing.
Nor did he originate 'finger in ear' which is probably millenia old and was common all over the world - possibly introduced into the revival by A L Lloyd as a a device for staying in tune (I have noticed that those who take the piss out of it usually can't sing in tune)
Jim Carroll