You got mislabeled the name "your own", so there's no solid evidence that the Freddy sprite in the OMC minigame is Golden Freddy. It could just as easily be Michael himself, who has been associated with Freddy in the past.

Hell doesn't have to be a specific location. A realm of torment and suffering can still be considered "Hell", especially by the one experiencing it. And who's to say that William ISN'T in Hell, and that the Vengeful Spirit followed him there to make sure HE'S the one in charge of the torment instead of Satan? He could leave the realm of torment any time, but doesn't because he wants to see William suffer personally.

There's no evidence that OMC is in direct control of William's Hell, he could simply be guarding the entrance to the lake (whether it be the gateway to Hell itself or to a higher plane, etc.)

The OMC minigame can easily be explained under WillHell by the Vengeful Spirit keeping William Afton trapped in a part of Hell, one that HE gets to customize rather than Satan, and OMC finally convincing him to pass on and leave William to rot alone.