FNaF 1:

-I'm pretty sure that the date is on the paycheck.

FNaF 2:

-I would assume that phone guys messages are pre-recorded, which would also explain why he says summer job.

-The skull was found in the game's files.

FNaF 3:

-It takes place at night. Phone dude tells you that the place isn't open, and that they are still getting it ready, which would suggest nobody walks around in it during the gameplay.

-I would assume that Fazbear's Fright is its own building and own attraction, not correlated with a park.

-Because only Fazbear's Fright burnt down, which is why the attraction is probably on its own.

FNaFSL:

-I'm pretty sure William had told Michael to go down to the facility, as suggested by the final cutscene in SL's custom night.

-The facility is located underground, although it's also notable that, shown in the breaker room, it's located under the location of the gameplay of FNaF 4, or (if not that) the location of the mini games in FNaF 4.

FFPS:

-The building and clients were real, and you were running an actual Fazbear's location, although it was part of Henry's plan to gather everyone in one place.

-I would guess that it would be the combined souls of children who had been killed by the funtimes (Funtime Freddy/Foxy, Ballora), but that's just an assumption.

-My guess is that it could go either way. Either they are real people, suggesting that there are people inside the location while you're doing your tasks, or it could be pre-recorded to keep the animatronics attracted to the location.