I'm using the condition GetQuestCompleted for the first time. I'm assuming that "GetQuestCompleted xxxxxx != 1" means to check that the quest IS NOT finished.

Yah or Nay?

Yah.

And I'd like to know the difference between "!= 1" and "== 0". There must be some subtlety at work there but I can't get it.

Beats me. I assume they are the same but could well be proven wrong somehow by a smarter person.