It is pointless quoting the '54 definition which is largely based on Sharp's work if you have rejected the conclusions that that definition was based on

I don't want to go too far down the road of angels and pinheads, but 1954 extended Sharp's concept to include songs with a known composer - provided they had been absorbed and refashioned by the community. Roud as far as I can see is saying no more than this, although there may be ambiguities over what constitutes 'refashioning'. Those light opera songs you mention were never going to be the kind of songs Walter's relatives would sing down the pub.