Let's say player A and player B are equally good, but player A gets twice as much powerplay time. Do you think it's reasonable to expect player B to have as many powerplay points, or even total points? I would say no. Now let's say player A is better than player B, and plays with better linemates at 5v5 and against the same competition. Wouldn't you expect player A to get more 5v5 points than player B? I would, which is why it doesn't add up that Wheeler isn't out producing Marner. it must be because he's not actually better
I agree with you on 5v5 vs. PP.
I still think people underestimate the 'luck' factor. I put luck in quotations because it is not necessarily luck, but something that is of low probability to re-occur. Marner's OIS% coming into this season:~9.2 (was 9.6% two years ago, 8.8% last year), Marner's OIS% this year 14.2%. That is an over 50% increase. Yes a lot of that can be explained by being moved onto Tavares line and in general NHL scoring increasing this season, but some of that is luck. Maintaining this OIS% will be incredibly difficult, it is far more likely he regresses towards 11-12% then it is he keeps a 14.2%.
Wheeler's being at 11% is still on the high end for his career but it is much more reasonable. If you gave Wheeler Marner's OIS%, Wheeler would be producing just as much as Marner 5v5, or at least a lot closer.
The difference between the two is marginal at best and there are arguments for either side. I expect Marner to end the debate within a year or two though.