"How do the core musicians - say melodeon, bass, drum, guitar arrive at the appropriate tempo?"

The "appropriate tempo" depends in part on the nature of the audience. You often need to play at a slightly faster tempo for inexperienced dancers, as they will usually just walk or run through the figures - more experienced dancers like to have a bit more time to put in some stepping or fancy moves. So you need to pay particular attention to the first few dances to see how they respond to the music.

You can also get a feel for the dance from watching the walk-through, and this can give an idea of the sort of rhythmic patterns and helps you to choose an appropriate tune for the dance.

If you're still not sure what speed to start off at, ask the caller. And keep watching - the caller will soon signal you to speed up or slow down as necessary, and you should be watching the dancers anyway.