You are stuffed as far as that character is concerned as an opener - either you or someone in a MP game has killed the CK and got the quest.
Your only option is to use another char to open the level for you - the bit of good news about this is now you can kill the CK as much as you like and not ruin the quest for someone else!
I don't believe anything has changed with the dynamics of the CK - it works as it always has except you have to kill Baal to do it not Diablo as before.
There has changed something, before patch 1.10 you could kill the cow king as often as you want, now you can only do it once per difficulty.
Anyway, the note on the cow king is dead is still present when you change the difficulty under the quest list, right? I just find it odd what she says, "I can't do that yet" doesn't sound like something sensible when not being able to open it due to already having killed the king (only sensible when not yet having killed Baal on that difficulty), I thought that when trying to transmute things in the cube and not being able to the characters don't in general say anything.