I have found a bug that will allow a player to enter any inferno level.

steps used to find bug.

step 1- finish hell difficulty

step 2- click "change quest" in you game select screen and set difficulty to hell. Then choose the area you would like to play

step 3- click "public games" and the area you previously selected will now be playable.

This may work for skipping areas on other difficulties, but i have not checked.