The problem is that even the erudite statements made by Stringsinger above would not be agreed by everyone. That's just his view, and of course he is entitled to it. But he should not expect everyone else to agree (and he probably doesn't!)

The trouble starts when people give their definition of folk music as fact, when it's only their opinion. That's why I said, it's best not to try too hard to define it, because for every one person that agrees with you, there will usually be several who don't.

I think it's true to say (IMO of course!) that certain types of music or songs would be agreed as being folk, by most people (but again, I expect not all!). Then there are lots of other types of music that some would call folk, and some would not. The line between the 'generally agreed' part and the 'not generally agreed' part is difficult, if not impossible, to define.

For example, many people would consider Guthrie, Dylan and Paxton as folk singers, and (some of) their songs as folk songs, unlike Stringsinger.

If Bing Crosby or Elvis sing a folk song, is it still a folk song? I think so.

Yes, I agree. The song is still a folk song. A 'pop' performance of a song doesn't change that fact (IMO!)