After piecing what I could together, I think I may know what happened to Lu Ten! He wasn't killed in battle, -Koh stole his face! Iroh abandoned the army and journeyed to the Spirit World to get him back, but he was too late. Lu Ten was gone. That would explain the Avatar Extra's rumors about the strangely familiar face Koh used, and the one where it says that many people belive this (after Lu Ten "died" but before he came back to the Fire Nation) was when Iroh jouneyed to the Spirit World, and partly why Iroh can still see into the Spirit World.
I'm still working on it, and it has some holes, but I think it's fairly solid.