The sound of the bell is an ultra high sound.As a person ages, they become unable to hear the most high-pitched sounds anymore. So that would explain the adults not hearing the sound of the bell. As for why the main character hears it even as an adult, let's say his hearing ability is superhuman.
The Hobo is JesusPerhaps he's a ghost, which could tie into the Holy Spirit. Perhaps he died, but now lives. Jesus was an unshaven man who had no belongings, was disbelieved by many, and shared a lot. He also saved a lot of people. The Hobo has no belongings but the very basics for survival, and yet shares what belongings he has. Most people don't believe he exists. He's unshaven and dirty, since he's a Hobo. He saved the main character by sharing his "joe" so the boy wouldn't freeze to death, and he saved the conductor. Also:
Billy is Dewey.The reason that Christmas "doesn't work out for him" is that Malcolm, Reese, and Francis have always ruined Christmas for him. Lois hasn't been shown to completely cancel Christmas before, but she's threatened it. Maybe in this Malcolm in the Middle / The Polar Express world, she HAS completely canceled Christmas. Besides, the two have an uncanny resemblance.
The Hobo is Santa Claus himself.Santa can take on a completely different appearance, and he can be in two places at once. He appears first to the Boy as the Hobo because he knows that the Boy won't believe his true form early on, and is preparing him to believe. This is either weird or awesome, your call.
The movie takes place in the same universe as Galaxy Express 999The North Pole is not on Earth, it's a different planet entirely. The fact that, for several hours, it's five minutes to midnight refers to North Pole time, which runs slower than Earth time.
A true visit from Santa is a once-in-a-lifetime experience.This is why the poor boy never received a gift before the events of the movie, and why the parents don't wonder who puts the presents under the tree. He doesn't deliver to every house every year, but only to a lucky few. Most of the time, the parents put the gifts under the tree. The poor boy's parents didn't do this, so that's why he never got a present before his turn for a Santa gift came up.