There is also a question as to whether one good song constitutes a good songwriter.

Interesting question, but I would say 'yes'. Even if a songwriter only ever wrote one song, and that song is generally agreed to be 'good', that makes that person a good songwriter in my view.

I suppose if they wrote another 1,000 crap songs it makes it seem like on average they are a crap songwriter, but in my view the one good song is enough to outweigh all the crap ones, since the latter will be ignored anyway.