A breakdown tune is used for the speedy bit at the end of a dance - when everyone has got the moves and you want to signal to the dancers the dance is coming to its end. It does not work if the base or the drummer ignore this and continue to play at the starting speed.
In morris dancing the speed often changes - sometimes in the middle of a bar. With sword dancing there is often a walk on speed and a dance speed (s). Again- this can be ruined by a well meaning but unpractised muso who wants to join in.
Establishing a lead instrument sounds a good idea - maybe the only one with PA if the band is large. The caravan and camping club use this idea with often quite large scratch bands. It seems to work well. Encouraging others to be the lead instrument in turn also works well.
The trouble with practising a tune is that often in a music session you would play it faster than you would for a dance.   It might be best to have set dance tunes and always play them at dance speed.
Childrens dances need to be faster then adults - they have shorter legs.
The most difficult thing is if you have started too fast ( or too slow)for the ability of the dancers and you want to slow down or speed up a little while playing. It happens to the best of callers. Its worth practising this with the band. It should sound seemless.
Enjoy
FloraG