When responding to the first call, when they show the interior of the truck, it's clearly not Otis driving it, but a stand-in.
Juvenile criminal records are sealed from public inquiry but can be used for several legitimate purposes such as public safety employment. As such, they would have been discovered during his initial background investigation.
On the rooftop rescue, there are three firefighters and two paramedics. Severide discovers the man hanging over the edge on the cable and instead of using the staffing present to pull him up on the cable goes over the edge himself on a rope.