how can I delete a note or a rest in the middle of a phrase?
And how can I change the duration of selected notes without getting rests - i.e. make a four quarter note line a 4 eigth note line (double time)
Type I to turn on Insert Mode. This is the answer to both questions.
When you’re done, type I again to turn off Insert Mode.
Bear in mind that Insert Mode only affects the selected stave and the selected voice, so use it carefully.