Forget names on walls, mysterious teenage boys, and Ilana saying that the Monster is stuck in Locke's form. The thing from last night that I keep coming back to is the fact that BEN showed up in the sideflash.
After seeing LA X I assumed that sideflash timeline was a fork from the main timeline caused by the Jughead detonation. It seemed clear that the island had sunk in the Incident, and what we've been seeing is the result of a forked timeline from right then.
However, Ben was on-island when Jughead went off. He had not been evacuated. If the Incident sank the island, he should have been killed. Since he's up and walking, this means that either Jughead didn't sink the island, or the forking point for the flash sideways was BEFORE the Incident.
The young Ben we last saw had been shot by Sayid and had been taken to the Temple for healing. He would have "always been changed". That doesn't seem like the kind of person who would be randomly teaching European History. So, I propose that the sideways Ben is someone who has never been to the island, at all.
If it wasn't Jughead, what would explain the huge differences we're seeing between the timelines? I think it's that Jacob was not present in the sideways timeline. He manipulated everyone on his wall onto the island, and without him around, he didn't meddle in their lives.