By definition I think Mackinnon should win this one.
I would disagree. The reason Mackinnon is 43 points ahead of his teammates is partially because of injuries to their top players. Overall though the Avs have a very deep team so they could withstand injuries. The next 6 Avs top scorers missed between 11 and 27 games. Even with those injuries they still scored 236 goals (35 for Mackinnon) which was 7 shy of tops in the league, as compared to 223 goals for the Oilers (43 for Leon).
Comparatively the Oilers had 5 of their next 6 top scorers miss between 6 and 16 games but the effect was more noticeable for the Oilers when their top guys were out because of the lack of scoring depth. I'm not saying Mackinnon didn't have a great year but in contextual comparison to Draisaitl and the Oilers I think the "43 more points than his next closest teammate" argument is overblown.
Further regarding team strength compared to the Oilers:
The Avs had 10 players who scored at least 10 goals, whereas the Oilers had 8.
The Avs had 16 players with at least 20 points, and the Oilers only had 11.
They also had 4 defenseman (including Cale Makar with 50) who had at least 25 points wheras the Oilers only had 2 (Klefbom was tops at 34, and Bear had 21 FWIW).
Again I think this provides context when it comes to showing who was carrying how much of the their team's load offensively.
In terms of dominating performances I think Leon did more to win the Oilers games. There were many nights where if Draisaitl hadn't have been going the Oilers would not have won. Also, when it came to clutch scoring Leon and Pasta led the league in game winning goals with 10 apiece which is quite significant, whereas Mackinnon had 4.
The McDavid argument is weak if you watched the Oilers all season. McDavid had dominating games of his own but this year it was Draisaitl who pushed the team far more often, and it was more a case of McDavid benefiting from Draisaitl than the other way around which was good for the team with McDavid coming back from injury and not being 100%. As such, Draisaitl shouldn't be punished because he plays on the same team as McDavid and played with McDavid for half the season, especially given that his PPG rate went up without McDavid in the second half (you also can't fault Draisaitl for having chemistry with Yamo).
Also comparatively don't discount Cale Makar's impact for the Avs on Mackinnon and the rest of their forwards. The Oilers don't have a comparative offensive weapon from the backend to help drive offense (the McDavid auto-zone entry notwithstanding).
I think if you look at overall context then Draisaitl's performance this year is more impressive than Mackinnon's. I think Draisaitl was more of a significant factor in his team's success than Mackinnon, and I think that Draisaitl was clearly the Oilers' MVP this year too. To me that makes him more deserving of the Hart trophy. Mackinnon may be more of a runaway MVP winner on his own team because of the injuries but Draisaitl was still a bigger contributor to his team's success thus is a better league MVP candidate. Don't get me wrong I think Mackinnon had an awesome year but Leon was MVP of the league this year.
edit: added a few more points.