You get the follower option after you complete the questline, which I did. It's the same sort of thing as the Mage Apprentices and Dark Brotherhood murderers you can get after completing those respective questlines. Also, I just found something interesting. I decided to go back into the cave to see if I could find the dead body, and apparently, the dead Knight clone had respawned inside the cave! I saw him (now more of a caveman than a knight) completely unarmored, with an awesome looking staff, fighting a massive horde of goblins by himself. I killed all the goblins, then talked to him, but he had no dialogue options. Then I used the console to delete the clone. I went back to the Priory of the Nine, expecting the problem to be solved, but I still did not get the "Come with me" option. Also, the last backup save I have is... well... distant, let's leave it at that. :P