But that last part still has a place in this story and has to be dealt with. Thankfully, the last scene at the Talon really underscores the idea that Clark's feelings for Lana are not a healthy thing. First of all, Clark assures Lana he had a "dream" and not a "nightmare." People were genuinely outraged over this, wondering how come Clark didn't consider his AU experience a nightmare. My answer? Clark is not talking about the AU. This is proven by his next statement: Clark describes to Lana how he bought her a ring when they were ten years old to ask her to marry him. The interesting part about this statement? It never happened. Not only did it not happen in real life, it didn't even happen in the AU - Lana told him it happened in the AU, which is an entirely different thing. So why does Clark describe this to Lana as a dream he had? It doesn't make sense until you consider that the dream Clark is talking about is Lana herself.
Think about it. The Lana Clark sees in the AU is the Lana that we all thought we saw at the beginning of the series: wholesome, innocent, loyal, steadfast, faithful. That got blown in the first season when we saw her wavering between Whitney and Clark; it was incinerated in later seasons when we watched her read Chloe's letter, tell Chloe she doesn't consider her close enough to be a sister, waffle between Adam and Clark, Jason and Clark, now Lex and Clark... Lana was never AU!Lana. But at the beginning of the series we thought she might be, and so did Clark. He's seen her as AU!Lana all these years; the point of this AU-episode is that it's a metaphor for Clark being willing to twist reality to match his Lana-dream. But now, after his experience in the AU, he's beginning to realize that AU!Lana never existed in that world or this one... she was a dream of his from the very beginning, one he made up when he was five years old and gazing at her from afar through the telescope.
I think that's why Clark describes to Lana an incident that never happened as a dream... it's a way of indirectly saying to her that he's been in love with a girl for all this time and is only now starting to understand that that girl was never real. It's something he wanted to have happened... he wanted Lana to be that wholesome girl that's been in love with him and he with her since they were ten... but it never did, and more significantly, maybe the entirety of his interaction with her since high school has been no more real than that ring-giving incident, because the girl he was interacting with never really existed except in his mind. Significantly, Lana asks him what happened after he gave her the ring... and his answer? "I woke up." Clark is waking up, y'all. The look on his face after Lex and Lana walk off is one of sadness, but it's not "Aw, Lex got my girl" sadness - it's "Maybe the girl I love doesn't even exist" sadness.
As for Lana looking back at him... oh come on, are we really surprised? Lana's been on the arm of the guy she has and simultaneously looking at the guy she doesn't have for about six years now. This is just same ****, different day with her.