Help me with my heater!!!

I first would check for cracks and condensation. The sound you are hearing is probably the contacts opening and closing and is not normal. It might work fine for now but what may happen is the little arcs you are hearing may actually weld the contacts to the element and cause the heater to stick in the on position. I personally wouldn't trust it and replace the heater.

More modern electronic heaters such as the Aqueon Pro shut themselves off when left out of the water.

I would replace it. But that's easy for me to say. Perhaps if you explain that it's a possible safety issue to your dad he will buy you a new one. Maybe you can talk him into an early Birthday/Christmas Present.

You can you use it but keep a close eye on the temperature but that's hard to do 24/7. It may never completely fail but you are at a higher risk.

You should be fine, just keep a close eye on it and be on the look out if the temperature spikes. I personally would replace it, But then again I work for the military industrial complex and I'm really good at spending other peoples money

I can't really answer that. It's good practice to check your temperature at least once a day. Just to make sure everything is operating correctly and that a plug wasn't left out accidentally during a water change.

I use big digital thermometers so I can't help but check my temperatures several times a day.