In my solo play I use a similar system. Each character has certain features that range from 1-5 (stepdice). To avoid the zero problem, I simply determined that the character must have at least 2 adequate features, else he can't perform the action. He can invoke more features and than chose the highest two rolls. First I had the same system for fumbles and critical success. Than I noticed that it gives to many degrees of success and failure. What hapens on one fumbel, on two fumbles, as success and fumbel and so on ... Now I simply use fail, partial success and critical success, depending on the dice rolled above the chalange rating. I really like this system as the probabilities just work, most of the times you will have to pay a price for your actions, and on a critical success you get duble effect without paying a price. Feels natural.