Marner helped create a lot of offense and goals during the playoffs - even scoring some himself - but there's no point in going through every playoff goal again when you've already made up your mind that literally everything that happens while Marner is on the ice is either bad or because of somebody else. When Marner gets an assist, it's only an assist - why he no score goal? When Marner scores a goal, it was set up because of the other guy. When Marner makes a quality play that generates an offensive chance but doesn't get credited with a point, it doesn't matter because he didn't get a point. When Marner makes a nice defensive play, it was actually because of the goalie or defenseman and it's only defense - we can pay scrubs for that, right? When Marner gets points, hits, blocked shots, takeaways, and considerably tilts the ice in every way in a game, it doesn't matter because he didn't personally get a shot that one time.
I'm not sure there's actually anything Marner could do where you'd give him his proper credit.
Literally only one of those 9 things I named was an "expected number", and whether you want to acknowledge it or not, expected goals are a valuable metric.
You seem to think paying somebody their worth means an automatic series win. That's not how this works. Everybody's operating under the same cap. You pay players more to bring more value and impact to your team. Marner does, and is far, far, far from the reason we've lost in the playoffs.
And for the record, the GF/60 stuff was to disprove the false claim that was made about Matthews producing better away from Marner.
Long term, there's still more to evaluating GMs and coaches than looking exclusively at playoff series wins and losses with zero context. While that is a result that we as fans highly value, that is just one of many types of results, and one that is usually not actually very representative of much. It can get an organization into a lot of trouble if they put too much stock into it. For example, the likes of Vancouver, Montreal, Islanders, etc. held onto bad, destructive GMs and made counterproductive moves for the health of their franchise because they overemphasized a playoff run instead of looking at the bigger picture, where their franchise was at, and how those individuals were actually doing at their job. 3 and 4 years is also not long-term.