Why are Tim, Brian and Toby proxies in your au
Slenderman needed proxies, even if they weren't magical like Zalgo's he still needed some extra support against Zalgo's proxies.
He went after Tim since childhood, wanting to get him young so he could 'train' him and so he wouldn't know anything outside of what Slender taught. This obviously failed, despite the fact he did manage to get Masky to exist, he wasn't a permanent state, and instead of going after who Slender wanted, went after Slender instead. This, unfortunately for Tim, did not stop Slender, because Slender doesn't know when to give up and try again I guess.
I do not have an answer on how Tim avoided being a proxy for about 28 years, he's just cool like that and Slender sucks.
Regardless he avoided that all throughout Marble Hornets, but then Brian died. Slender took this as an opportunity, by taking Brian as a proxy (even if it takes a bit to convince him to actually do stuff for him) not only would he get 2 proxies instead of 1, but Tim would hopefully be more accepting of proxy status if Brian was there. (did not play out like Slender hoped but still) Tim was finally full on caught and put in the mansion 2 years after Brian did
Slender stayed with just Tim and Brian for a bit, sure they weren't exactly what he wanted in a proxy, not at all, but he still had them, and with the Slendermansion being made the need for a feral attack dog proxies were moreso pushed to the back anyways. And then Jeff joined. Jeff is 13, self centered, pushed his way into this mansion, and loud mouthed. Jeff would not listen to authority to fix his behavior. Technically Tim and Brian still were enough, but Jeff was annoying. Everyone knew that but Jeff himself.
So Slender went after a younger proxy, closer to Jeff's age, in the hopes that maybe if they were seen closer to a peer than authority he would listen. There was no exact reason to choose Toby himself, just like there was no reason to choose Tim over any other child.
