From A Different Angle
Well, most people who approach this topic come from a physical evidence based point of view, like the tattoo being missing and hints of the set designs. However I want to come from a different angle, Mike’s thought process from an emotional and fact based logic. Even though this is going to be a very short theory it is one that I want to make.
Right, time to cut to the point. Let’s look at Mike’s emotional based logic tells him that L is as dead as L in Death Note and Tim Kring’s Heroes. This is why it took him eighteen months (one and a half years) to mourn her. After all, it is quite clear that there was no change of Mike’s point of view as he is still moping around and missing her. He still is being clouded by emotions during all of this time, which is probably why he never was thinking about the destruction of the Upsidedown and the day that L ‘died’ as it would have gotten him all upset again. To put it simple, his emotions were clouding his mind.
The end of his emotions clouding his mind was the time before he went to graduation. Both him and Hopper were able to address the issue and he was allowed to truly get his emotions out. Once his emotions were flushed out he was able to get his mind clear.
Once he was free from his emotional thinking he was able to think clearly, so, once he saw the speakers that was when he was able to think more critically about the moment without his emotions getting in the way. Also, I am pretty certain that Mike would have seen many speakers in those eighteen months and not once did they cause him to think about that moment in a critical way. Yet now was the time when one of them caused him to think about it critically causing his mind to start ticking shows an internal shift.
That internal shift was that he was no longer thinking emotionally but he was thinking logically. He was using nothing more than cold hard logic, logic that was no longer clouded by emotions. It was not his emotions telling him that reminded him that she could not see her powers in that moment as her Kryptonite was activated, it was his logic. It was not his emotions telling him that she got away, it was reason.
After all, all he would need to do was just ask himself how? This would cause him to deduce the only way that the moment was even possible was if there was someone else, who could use her powers. Someone who could have pulled off the performance. This would have led him to deduce that it could have only been Eight. After all, who else was there that could have done it?
So he would have then asked himself how she was able to get out of there without being seen, this would have led him to deduce that she could not be seen, somehow. This would have led him to deduce this too.
So that is why I am in Team Believe.
(Unless Conformity Gate is proven true one day and episode eight really was nothing more than Vecna’s Infinite Tsukuyomi…)