I mean the Play outright suggests that if it wasn't for the Mind Flayer's fuckery in the 50's Henry wouldn't have developed his powers or become corrupt-

Is the play canon?

Yeah, The First Shadow is canon, marked as the official prequel with the duffer brothers having the story written by them and Jack Thorne and Kate Trefry,

Doesn't change the fact that Vecna controls the Mind Flayer. Even if the Mind Flayer was his point of origin, that doesn't make it the big bad somehow.

That's the thing, the Mind Flayer is the source of Vecna's powers, to put it simply: no Shadow - no powers and life for Vecna. In fact, Vecna ​​does what the Shadow Monster needs, namely, he kills, opens portals, terrorizes Eleven's friends and loved ones. And the theory that the Mind Flayer is Vecna's creation has already lost its relevance. Moreover, the play also said that Henry had a girlfriend, Patty. This already breaks the concept that Henry hates all people. And he began to hate them after the Shadow possessed him. Vecna's motivation and powers are nothing more than tricks and manipulations of the Shadow Monster. Also, Henry a priori cannot be universal evil, because whatever he is, he is by nature an ordinary person, but the Mind Flayer may well be world evil, because he simply represents a strong negative energy, with its own goals and intelligence.

Your theory is flawed at the source. You don't seem to understand that regardless of whether his origin is from a certain entity or not, his role in the plot is still the main antagonist. You need to examine the story itself to determine who the primary villain is, rather than trying to rationalize your faulty viewpoint through a technicality.

To be honest the duffer brothers THEMSELVES (allegedly accoring to the villains wiki of all places) that despite the retcons that the first shadow brought in, Henry/Vecna IS the main antagonist of the show-

Epic twist it’s either Lord Voldemort or Dormammu

But in reality it could be The Mind Flayer

It is also worth distinguishing between the concepts of the main antagonist, the central antagonist and the all-encompassing antagonist. Sometimes these concepts can be combined, but in the case of this series, Vecna ​​is only the central and main antagonist, I do not argue with this. However, as is customary for central antagonists, he mainly opposes the main characters, he himself is not a global threat. And the Mind Flayer acts as an all-encompassing antagonist, his goal is not to take revenge on the main characters, he does not hunt a bunch of teenagers, but to enslave the whole world. And with its abilities, Mind Flayer really is a global threat. Simply put, if you kill Vecna ​​separately, it will not be a victory, because the shadow will again inhabit someone else and make a new Vecna. To win, you need to kill the Mind Flayer himself.

That theory is entirely unproven. Your claim that the Mind Flayer is "possessing" Vecna lacks any sort of evidence. If anything, season 4 leads us to believe that Vecna is controlling the Mind Flayer.

You seem to think that the Mind Flayer has a mind of its own. Everything that it did was with Vecna behind all of it. Any intention that you think it had was, in reality, Vecna all along. It is Vecna who wants to take control of the world. His goal was never to take revenge on a bunch of teenagers, but was in fact to transform the world into a new Upside Down. If his intentions had been otherwise, season 4 would have played out a lot differently.

You only ever deal with speculation. Consider what is actually in the plot of the show itself, and work your way from there.