Somethings are yet to be discovered, and somethings are well hidden. I have a couple of theories that either you have thought of or not.
1) The four bullies ARE the nightguards:
I've thought this one through many times and made the conclusion that it was them.
~ The older brother is the Pink Guy - As you see in one of the FNaF 2 mini games, this particular pink character looks nothing like Purple Guy and is hanging out by Pirate Cove. The brother, of cours, wears the Foxy mask.
~ The boy with the Bonnie mask is Mike Schmidt. I made the conclusions by their clothing. And of course by Scott Cawthon's descriptions on these characters, yet Mike has hair (I'll get to that later on).
~ The boy in the Chica Mask is Jeremy Fitzgerald. He seemed to be one character that was younger than the other 3. He also seemed to be the shy type.
~ And Finally, Fritz Smith is the one with the Freddy Fazbear mask. His hair looks like an orange/blonde color. But another thing is his clothing: Green shirt, but blue shorts.
The one thing that I still have to figure out is who are we playing in FNaF 3...
2) THERE WERE 2 BITES:
~ The first one being in 1983, when the poor young child was thrown into Fredbear's gaping mouth. The reason we never knew about this bite in '83 was because it was covered up and never known of.
~ The second being in 1987, the one we do know of. Of course it was Foxy, biting off Mike Schmidt's frontal lobe. This one wasn't covered up.
3) Mangle in the Parents bedroom:
~ The reason for Mangle being in the house is because the child's parents work at Fazbear's. I'm thinking it was their turn to rebuild the animatronic, henceforth it being in the house and it also explains the Mangle's Quest mini game in FNaF 3 .
4) Pink Guy:
~ As I said before, it's the older brother. The reason we probably don't see him throughout the rest of the games is because he was caught and sent to prison. Purple Guy must've framed him due to the Bite of '87. Pink Guy was near Pirate Cove in the FNaF 2 mini game.
5) Springsuit, Shadow Bonnie & Freddy:
~ In FNaF 4 mini games, we see Purple Guy helping a person in a Springbonnie suit. Here, I believe that Purple Guy caused those two people in the suits to die. HE 'helped' them, so he must've made sure the spring locks weren't tightened correctly, causing the employee's deaths.
6) Plushtrap, Foxy Plushies:
This plush finger trap is suddenly added to FNaF 4. It makes sense since there are Foxy, Bonnie, Chica, Freddy, and Golden Freddy Plushies. The Foxy plushie is missing it's head and it could be due to the older brother. He seems rather obssessed with this fox, and scaring his younger brother with a Foxy mask.
That's it. The only things I have yet to theorize are:
- The clock in FNaF 4
- Balloon Boy & JJ
- The FNaF 3 security Guard
- FNaF 1 kitchen camera
- The box