You do not need to drink apple juice if you have been eating beets. In fact the apple juice is optional for many of us who are fighting a problem rather than just cleansing.
Only you know if you are strong enough or not. I can't make the decision for you. You can reduce the Epsom Salts by a third and the olive oil/lemon juice my half and still get a powerful flush.
A gall is short for gallon and one US gallon is equivalent to 3.8 litres.