I completed Durlag's before going to Ulgoth's beard. If I pay for the tour, will the tour and the demon be there?

I don't think so, no. But you might want to make sure you have the Soultaker dagger.

Just did it. The tour is there, the demon isn't. The tourist who walks onto the trap doesn't die and the demon doesn't appear.
This suprised me. I would have thought that if the game takes into account the fact that the demon is dead, it should also take into account the fact you have already been to Durlag's tower.