In the first film, Laura realizes that Scott really is Santa Claus because he gave her (back) the thing she wanted most of all for Christmas that year—he returned Charlie to their home. (That he took him in the first place is beside the point.)
All of the rest of the elves are cheerful, so why is Bernard so uptight? Well, given his high position and the legacy status of Santa, he's likely gotten impatient with giving the same spiel over and over again, in addition to becoming more and more jaded every time another Santa goes missing, which would no doubt be worse if he thought of the Santa in question as a personal friend.
A gay man, or one who'd simply be happy single, wouldn't be able to stay as Santa Claus for more than a few years unless they entered into a marriage they didn't want.
How many people before Scott has played the role of the Santa Clause, and how many times has someone been replaced after fallen off a roof or gotten seriously injured in other ways that tragically ended their clause? Assuming there was more than just one Santa Clause beforehand. The elves are pretty quick to accept that he's the new one, right after the last one's death. How many times has he passed that they're that used to it?
If being Santa makes Scott immortal doesn't that mean he'll eventually outlive his son Charlie? Charlie's younger half brother Buddy Clause would end up watching his older brother grow old and die while he presumably lives on to take over as Santa one day.
What happened to the previous Santa's wife? Did she lose her immortality and just crumble to dust when the previous Santa died? Risking your own life is one thing, but the realization that, after a certain number of years, your wife is doomed to die with you would eat at you and make you paranoid.