I am currently stuck in Act II Inferno and sometimes I try to get further in the story. Recently I killed Zoltun Kulle and talked to everybody quest related afterwards.
Then a friend asked me if I could help him in Act I, so I switched my quest.
When I came back to my game, the last available quest was "The Soulstone Chamber", so I had to kill Kulle again.
Now I am at the last quest for Act II where I have to kill the lord of lies. I am stuck and want to farm for better equipment, but I do not want to do everything all over again.
So basically what I am asking is:
When will quests be really finished, so that I can switch quests and never do the last one again?