I would think that the slow/fast thing would be a reaction to the music or part of it?
No. Well, maybe until you progress beyond beginner level and are able to lead confidently.
Yes.
Any time you like and any way you like, as long as you're doing it for a valid reason and not just because you can. i.e. the music demands it, or you've picked out a piece of the music that it fits with.
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