Wait, Sara and Marielle are they keys???
Okay, this theory is wild, but hear me out - we know from the most recent episode that fear can definitely be manisfested - the dolls were a man's fear. It was revealed that marielle hears the screams of the dead and that they cannot leave. We also know that jade is killed by the townspeople every cycle, and that Tabitha's fate is "worse". We also know that the original townspeople made a deal with an entity to gain immortality in exchange for sacrificing the children.
Now, let's look at the boy in white. He helped Victor when he was a child. He reappears to adult Victor in the current timeline. Who else has seen him? Ethan, Tabitha... and Sara. Why Sara? Tabitha is Ethan's mom. Tabitha as Miranda was Victor's mom. What if one of Tabitha's other incarnations was the BIW's mom? That would lead us to believe he shows himself to those tied to her.
Julie can story walk, so I won't count her - she has her own story - but what if Sara is Victor's sister reincarnated? That would explain why she saw the BIW, and the closeness she had started to develop with Victor at one point. And if she can reincarnate, AND we know the dead are trapped, what if THAT is the immortality? Their souls are eternally tied to fromville, which is why they come back each cycle. That is exactly the type of trick an evil entity would play - you get immortality, but not what you imagine. Instead, you get tortured for eternity.
Tabitha and jade are the exception. They tried to stop the sacrifices, and that is why every time, they get killed by the townspeople. Their immortality is tied to the children - the screams for help and their desire to be saved was manifested through the faraway tree they were sacrificed under. But each time they start to remember, they are both killed - knowledge comes at a cost.
So the people are stuck in an eternal loop of torture. The smiling monsters are not the townspeople, but they were the fear of one of the dead townspeople. The manifestation of fear explains why there are so many entities there, and why the smiling monsters were, as Martin stated, "just the tip of the spear". The dolls, the music box monster, the cicadas, the spider, maybe even the MIY - all manifested fear. And until Jade and Tabitha can rescue the souls of the children, none of them can be released.