What then constitutes a traditional singer is indeed the question, and I started this line out with that question, and it's a very difficult one. There's a lot of room between someone who only sings to himself in the shower, and a full time professional singer of traditional songs. Can we make an approximate dividing line to come up with a traditional singer?

Let me kick something in for a starter, and see what develops. I would take a traditional singer as one who learned his (old) songs from someone else (not directly from some book, broadside or chapbook) and sings them to himself or an audience, but someone who usually does it for free entertainment for the audience. I wouldn't exclude some singing for pay or other remuneration, it that wasn't the primary occupation of the singer.