Thanks for the feedback.
I'm afraid we are not currently working on the game anymore. The idea of only having a single line (and no eraser) was part of the game jams theme, and we're quite happy to keep it that way.
Difficulty spikes/drops happen because we did not have the time during the jam to let people playtest the levels and rank them by diffuculty.
If ever continue developing the game things like level ordering and level selection will be some of the first things we do.