Hmm
If I had to solve problem like this - maybe I would try to highlight cells, or path as you wind up, but make every next cell more transparent so that player gets the idea of the controls and timings in the beginning of each level, but then it's up to him to memorise? And you could slowly make the path decay stronger with each level
And about the length.. Maybe there isn't much to do as the bottleneck is the player's memory and I feel like it would begin to become really hardcore for a lot of players.
Although you could just make "easy mode" highlighting all the path, requiring the player to still keep in rhytm