So I've got all three words of the shout, which I unlocked with dragon souls BEFORE I summoned Durnehviir for the first time. Now the shout won't work. I'm pointing it at the ground and everything. Is there any workaround, using the console or otherwise?

I did it like you and unlocked all the shouts first. And it didn't seem to work, sometimes I would get a small noise and see a little blue swirl like you see at the beginning of a summons and then nothing happened. What I found through trial and error is that it isn't enough to point at the ground. You need a wide open space with no obstructions and enough room to hold a dragon. It might help to aim your shout a little bit away from you. I would recommend that you summon him outside of combat so you can get a feel for the amount of space you will need without being distracted by someone hitting you with a battleaxe. I can't help you with console commands to advance the quest because I don't know them.

Same problem here, I removed the save file and started a new game.