Just to pick up on a few points:
Using a "start note" rather than identifying a key comes to the same thing for a simple melody. if you always start on a particular note, you're always singing in the same key. The difficulties stare when you're singing in harmony with others, as you may have two or more different start notes. Once you know they key and get a chord or arpeggio of that key, everyone can find their start note.

If Dylan insisted on having a harmonica in the correct key rather than tune the guitar to the harmonica, this would be because he needed to have the right key for his voice, which would otherwise have been out of its optimum range (I know he always sounded as if this was the case anyway ..)

It's got nothing to do with "Celtic tonation", which I think is a meaningless term - Riverdance etc were played/ recorded largely in the same tempered tuning as is used for most other Western music, and indeed in a more highly processed and synthetic form than much traditional music. It's fast and metronomic and loud and tuneful - there's nothing remotely mystical about it.

Marje