Ah, but what if you started the song in F# and they have a G harmonica?

Most singers who do not accompany themselves with an instrument pitch a song according to the range of their voice and the state of their nerves. It may not be in the "proper" key - "proper" as defined by some bugger with a D/G melodeon or a harmonica.

I have had to abandon a song (only once) when I could not compete with the insistent G from an uninvited melodeon.

And as for the speed thing... there appear to be only two universal rules in folk music:
1) Play as fast as possible (never mind what speed the tune was started at).
2) Sing as slowly as possible (never mind what speed the song was started at).

The only exception is if you are singing and accompanying yourself on an istrument!