Sans is human, or at least mostly, so that means humans can use magic, yes you heard me correct. At the beginning of the game it says,"Humans sealed monster underground with a magic spell", this proves humans use magic.

Is Sans a human in disguise?

tl;dr: Sans is not a human, but rather a skeleton with some determination up on his bones.

Does Sans have 1 health?

With so little known about Sans, it's unknown why he only has 1HP storywise. In regards to the game's code, his HP is not needed to calculate how much health he has left. In fact, the health bar isn't even drawn because the code sets drawbar to 0, meaning false.