Where is the priest Konahrik? If there is one, it's most likely the "leader", or rather, highest ranking priest.
But why is his mask protected by that dragon head and requires all the others to be accessible.

Ans who wears the Wooden Mask?

That, sir, is the Million Septim Question

He was possibly Pope of the Dragonpriests.
I'm also of the belief the wooden mask, as a fake to try and open the final. A treasure hunters folly.

Konahrik is unknown. It's likely that he/she was the leader of the Dragon Priests, but was unable to receive their mask in a ceremony before the barrow at Labyrinthian fell and the mask became irrecoverable. So, they had the wooden mask crafted for time-travel to the barrow shortly before it was overrun and/or the altar was destroyed. From there, they could perform the ceremony, either solo or with the other dragon priests if the wooden mask is the last of its kind.

Obviously, it didn't work out like that.

Konahrik seems like it's the greatest of the nine masks, perhaps the idea was that if the Dragon Cult would be in danger and only one Priest would remain, he could use the rest of the Masks to increase his individual power to ensure the Cult's survival. It may have been just a tool, only to be used in cases of emergency.

In the trailer for the upcoming Dragonborn DLC it looks like Konahrik MIGHT be the main antagonist. You see a flashback to the mask barrow you get to with the wooden mask and every dragon priest there OTHER than Konahrik is accounted for. My guess is that the DLC will flesh out his backstory.

Konahrik is there as well, watch in 720p, fullscreen. You can see the mask rest in the Dragon's skull.

I believe he is the priest the others made fun of, considering his mask is garbage.

What? The mask was still there. But I do agree and think that Konahrik was the first dragon born too.

I didn't mean that his mask was unaccounted for, but that HE was unaccounted for. Other than the mask the game leaves open who/where/what he is, whereas all the other dragon priests that have mask spots there can be found, fought and killed.

Or....

Konahrik was expelled, the other Dragon Priest decided to stow away the only indestructible evidence that he existed (his mask) behind 8 locks within a Temporal realm only accessible with another key.
The first 8 were their own mask which they surely assumed (incorrectly) that they would never be defeated and therefore the 9th mask would never be discovered.
The tenth mask (wooden) was possibly nailed to the face of the banished Dragon Priest as part of his punishment, they weren't a popular cult and plenty folk would like to throw sand in a fallen ones face no doubt.
Furthermore by nailing it to his face he could enter the temporal realm where his mask was stored but never leave again as he couldn't take the mask off, this ritual may have even been fatal (possibly intentionally) for him and now he wants to come back and make Bethesda lots of money for his sins.

I too am interested in where the Mask was found and how it got, also to find out what really did happen to Konahrik even if he is actually not part of the Dragonborn DLC.
Maybe an old book, scroll or Bard's tale would suffice.