Certainly by the end of the story I don't think we're supposed to be sure that Luke "falling to the dark side" was more than an act that Palpatine sort-of didn't really fall for.

I think Luke investigated the Dark Side, maybe put himself in a position to be seduced by it, but never really strayed from his mission to infiltrate the Emperor's confidence, learn his secrets of immortality so that he could take him out for good.

At least that's how I made peace with it. If I'm supposed to think that Luke really did turn dark, then your argument is 100% valid.