In this case there is another task after the one for the diviner, so you're better off doing the shift click directly on the diviner to complete just that task, rather than on the quest as a whole.
Well, there has to be a better way to deal with this. I completed the diviner part with the admin book, clicked save all, then completed subsequent quests including the gaia fight. Figuring I was done with looking at the editor part of the book, I turned the edit mode off in my config and restarted my game. It reverted all my progress as if I never completed the quest with the admin book.
Without setting Edit mode, you can't just go into LAN cheats mode and use the admin book as clicking on the one item in a quest while holding shift does nothing to complete it. You can complete the whole line by clicking on it though.
I'm obviously doing this wrong but I'm going to have to go 'complete' several other quests again via the admin book because one of them was kill the ender dragon which can not be done again.
Perhaps the programmer of HQM could be encouraged to have a different way to complete errant quests or a clearer set of instructions on how to do so.
Kz