Apologies if I end up going off on a random tangent, but I'm mostly trying to address points 3, 4 & 5.

FNAF3 has a plethora of mini games which all have a very common element, hidden rooms. Which aren't part of what you see, you can complete all these challenges without really leaving. (golden freddy and bonnie secret stage being the main exception in that you glitch out of the room boundaries to exit) To actually complete them you find the hidden exit panel in a wall and end up in a secret area with the crying ghost child.

There is also shadow bonnie which enables the secret room theory, suggesting the reason the children's bodies are never found is that they are hidden behind false walls in fazbear establishments.

There is also nothing to suggest that there aren't even more children that were killed by purple man. In the mini Atari games in FNAF2 It just so happens that purple man has killed the number of children involved by the time of FNAF2, and simply gathered them in one place in some sick and twisted arrangement for foxy to 'entertain'. It also shows the original murder/kidnapping from back in the golden freddy / golden bonnie days (which would sadly debunk purple man being the victim of a bite causing the murders, as he'd already be a sick child killing scumbag with the original diner. But it could still be the reason every fazbear establishment is forced to shutdown.)

The atari games in FNAF3 suggest that nothing weird has occurred since FNAF2 and FNAF1 is the first event of this kind to happen in years, (likely because of a highly disgruntled employee who who has worked his machinations to get the establishment closed down as you made mention.) purple man who knows of both incidents, learns of the re-opening and [my theory is purple man is there on night 4, hence phone guys death, the animatronics going for you twice as hard as on previous nights, the souls can sense his presence, and genuinely think it is you, especially foxy who's animatronic was witness to the dead bodies of the kids] he tries to put an end to it by using the spring trap mechanism the phone guy mentions in his archive tapes about the endoskeletons. So uses the secret room in order to sneak and destroy the animatronics thinking its the end of the problems. (Subsequently getting the security guard fired for tampering with the animatronics on night 7 maybe? but unlikely because the main doors are boarded up suggesting purple man comes back a week or two after the events of FNAF1) However purple man gets haunted by the ghosts of 5 of his victims and thinks he can hide in the golden bonnie costume which was discontinued along with golden freddy because the mechanisms inside were so delicate that if you so much as breathed on them while wearing it the mechanism would spring back and kill any occupant. Which it did. Thus the same curse/haunting event applies itself to the purple man in the spring trap costume. The spirits of the murdered children are avenged and pass on, leaving spring trap.

But we learn of a 'sister site' a mini game shows a possible 4 different children wearing a crocodile, pig, cat, dinosaur masks? in one final mini game suggesting if a Five Nights 4 ever exists it won't necessarily be freddy's as by the real ending it suggests the curse is lifted off of the original animatronics (in reality it allows scott to use other new animatronics which could be scarier or freakier)

As for the phantoms, if it's a curse of some sort, then I'd assume because purple man is the origin of it, that it's a projection of his guilt. the guy worked there and if he even temporarily hide bodies in the costumes he knew what victims he put where, and somehow as his animatronic suit comes to life he can project them upon you.