Don't Believe Their Lies and Rewind
There is a lot of emphasis on ifs, so we should already be suspicious on that principle alone as to whether Lucas's theory is right. However, I just watched Season 5 in reverse order because I'm on my Memento-ST analysis arc and if you (be kind) rewind on your rewatch you'll see clear as day they got the date wrong. The day this all started—literally, as it's the first thing we see—was November 12th. Vecna does not say "we can begin" on the 6th. That's why he stole twelve kids, that's the biggest clue to the right date.
Both Stranger Things and Memento's plots are about erasing memories of trauma while creating a new fabricated reality to cope, trapping one's self in a loop or cycle; moreover, the ending mirrors the plot twist in Memento too (iykyk, but it concerns a death).
The fact the party gets the date wrong makes it clear the cycle did not in fact end. That hopeful idea is Kali's (and Mike's) illusion. The cycle of reincarnation never ends in the Curse of Strahd either (e.g., Ireena/Tatyanna = Will/El). This curse premise is the final D&D game Mike plays, and is what the entire season was built from, says Matt Duffer:
"It was early on in the writers room when we cracked Mike telling this story via the D&D campaign and the “I believe” moment where it really cracked open. So even though she was never there, once we came up with that, we’re like, “Well, now we have our ending,” and the rest of the season was really built from that moment on. We knew we had an ending that we felt confident in, and then it was just trying to build to that moment." (x)
"Don't trust her. Don't believe his lies." "She was never there. I believe." Who lies or whose lie is debatable in Memento (spoilers), just like how Mike and Vecna both tell lies to cope with loss and guilt. For Mike, the guilt is growing apart from Will after pushing him away represented in the loss of El, who she embodies in spirit and disappears on the 12th of November.
Vecna should have chosen the 6th to complete his full circle plan as that is when he presumed Patty died (who parallels Will/El) and massacred little Alice (paralleled by Holly) while possessed. That's the reason he chose who he did to restart his life, what's left up to "interpretation." Yet, the date matches Mike's most traumatic flashback in the finale.
So to me, it's not debatable. Vecna is a creation in Mike's head because of who the date really belongs to. Mike is caught in a memory wiping loop to cope with tragedy, which I theorize/headcannon to be a take on Stand By Me's ending, loss in retrospection and childhood goodbyes, where instead of a stabbing there is heavy emphasis on an AIDS allegory in relation to Will. I might just end my shit after thinking about that for too long.
Lastly, the "date" is missing at the end... the queer date between Robin and Vickie. The wrong dates, Lucas and Max's, Hopper and Joyce's, Will and Epilogue boy's, were instead chosen for this reason: conformitygate, lies, and the culpability of heteronormativity.
