Yes I have played many sports, and no, what Tkachuk brings is not difficult to measure. No, Tkachuk does not have a secret magical intangible quality that helps his team win. That's just what people say when they like a certain style of player, but they're not as good as they want. Tkachuk is all offense, while Marner actually does bring more outside of production. Helping generate adrenaline and rallying teammates can be a minor positive quality, but you make the mistake of thinking that only players like Tkachuk can bring that in only one specific way. That is achievable through many means, and it's something Marner does as well. If anything, Tkachuk is more prone to creating the opposite effect.
Ignoring opponent quality and context in the playoffs is indeed ridiculous. You're right about that. The mistake you make is thinking that later round automatically equals tougher opponent or increased difficulty to produce, and that's simply not true. Over the past little while in the playoffs, the Leafs have faced some of the most difficult situations to produce in the league. What we have seen is a team-wide phenomenon, not individual.
Linemates? But his linemates have produced even less in the playoffs. Heck, sometimes, the 3rd linemate may as well have not been there. Is that you acknowledging that playoff production does not perfectly capture the quality of a player? Because that's true, but destroys the whole argument against Marner. Ironically, his linemates are the ones who have been unable to convert his many good setups; creating the source of the criticism against him.
The difference between their cap hits is only 1.4m, and quality relative to cap hit is a different argument than quality.
Both Tkachuk and Marner have shown that they can lead their team in the playoffs. Marner has had plenty of good playoffs. More than Tkachuk, in fact.
Running him out of town is very much saying that he can never turn it around, which even if you believe all of the things you said about Marner, is a historically unsupported opinion. We saw that with Tkachuk himself. What's tiring is watching people single out and attack one of the best players this team will ever have, for 5 years, for no good reason, and advocate for destroying this era because they haven't yet gotten what they feel entitled to, as they refuse to understand and acknowledge the actual reasons why.