As both the mother of a dancer and a dancer myself, I can tell you that the whole musician communication thing can be quite complex. It is true, in my experience, that dancers do not pay an awful lot of attention to what music is used. The choice of tunes is usually at the whim of the teacher, and we need to be able to do the steps no matter what the tune. Consequently, dancers, especially young ones, really have no clue about speeds.
There are two hornpipe speeds and two jig speeds. You are right that the slower hornpipe speeds are for more experienced dancers and they do then get to put a lot of steps between beats. So the musician falls asleep and the dancer kills him/herself to get all the steps in. At a recent feis, my daughter danced a slow hornpipe to music that was not only fast, but kept speeding up as it went. She kept in time however and managed a third in her competition. The jig speeds differ based on the shoes they are wearing. In fact, there are really 3 speeds, one for soft shoe, and then a fast (traditional) and slow for the hard shoes.
I see more and more musicians with metronomes, not ones that make sounds but the show the beat to help them keep time. It must be really difficult for the slow speeds, because it doesn't seem natural to play that slowly. As for when they finish their dances in competition, usually the girls do two steps, right foot and left foot, so that means 32 bars. In the championship categories, they do three steps, so that would be 48 per dancer.
Ken, the kids and are doing quite well, keeping very busy with school and Irish dancing. This forum setup has really freed up time for the other family member, so I think it's great.