b) The datasheet says I can put zero in OCR0A when I want maximum frequency output. Why does the minimum useful value seem to be 4?
You can use zero which gives a frequency of 8 MHz (because it takes two toggles per cycle):
I've used a similar bit of code to generate clock pulses for programming other processors so it can't be too dirty. The ringing I suspect is partly an artifact of the measuring. This is only a 60 MHz scope.