The Blues are outplaying them, but Lethtonen gave them those goals, including the disallowed one. I think if Niemi had started, it would be closer, but STL would still be up.
As far as the offsides rule, yeah the right call might have been made but the interpretation of the rule is becoming exactly like the crease rule was in 96-97. There were playoff games determined by crease violations when a minute fraction of the toe of an uninvolved player's skate was hovering over the crease line. Here we have a playoff game seven getting paused and possibly derailed because of a longshot coach's challenge that turned out to be correct by 2mm. In hindsight it hasn't mattered much in this case because STL is taking it to them, but that was not an egregious offsides, and those couple millimeters didn't have any real impact on the play. If Tarasenko had gotten a clearer path to the goal or somehow beaten a defenseman BECAUSE of the offside step he had, then yes, it's egregious offsides and deserves review. This felt like Dallas grasping at a straw to staunch the bleeding that happened to work. For 2 minutes.
I dislike the rule, even if the right call was made.