What does "lighten up" mean?

Does the meaning stay the same even though I take out "if" from the first sentence?

"If" can not be removed from the first sentence here. "If" here is a conjunction that means "whether". Ex: The weather will determine if I will go outside today. Ex: He doesn't know if she loves him or not.

Why would the speaker say "into" instead of "onto" here? Didn't Arthur sneeze on the surface of the palm?

It is actually better to use the preposition "into" here since his hands surrounded his nose and enclosed the sneeze. He may have sneezed on the surface of his palms, but usually when someone uses their hands to sneeze, they cup their hand to enclose the sneeze. Therefore it is more common to use "into" in this context.

What is the meaning of “pack” here?

"Pack" in this context, means a close group. Mowgli belongs to the wolf pack. He is considered family to the wolves. Ex: The wolf pack stood around the water.

What's "I'm about to" mean?

"I'm about to" means you're going to do something very soon. Ex: I'm about to leave for work. I'll probably go in the next five minutes. Make sure you're ready on time. Ex: I'm just about to go for a walk. Do you wanna come? Ex: She's about to start her exam.

What is the difference between "mustache" and "beard"?

The difference between a "beard" and a "mustache" is that the "beard" is facial hair that grows on the chin and cheeks. While a "mustache" is facial hair grown on the upper lip only. Sometimes the two are connected on the face. Ex: A. I'm thinking of shaving my beard! B. Really!? What about your mustache? A. Oh, I would keep my mustache. Ex: I can't grow a beard. I don't grow enough facial hair.