I mean, I suppose from one perspective she's keeping Lex from completely going over to the dark side, but would he be there if he weren't so alienated from Clark?

And I don't even think it can be said that Lana's keeping Lex from completely going over to the dark side. I mean, he's pretty much already there -- 'cause hi, holding sentient beings against their will and experimenting on them -- so at this point, it's just a question of how dark is he gonna get before it's all over (i.e., will we actually get the patricide with Lionel before the series ends). Lana's not really doing anything to hold Lex in check (largely because of how expertly he manipulates her/how much she doesn't concretely know about his dealings); to the extent that there's the appearance that she's having some kind of effect, that's all it is -- appearances. Subterranean's awesome last minute and a half makes it pretty clear that Lex is going to do whatever Lex wants and in the end, I totally think he'd choose his ambitions over her if it became clear he couldn't have both.