What does "pompous" mean?

Why did he say "it"? Can I skip it?

He said "it" because "overworking it" is a phrase that means "overdoing it" or "being too extreme". Therefore, "it" can not be skipped here.

When do you use the expression "or as"? and what does it mean?

You use the expression "or as" when you are linking alternatives together. It's a piece of alternative information to give the listener more context to the subject. Dumbledore is informing Harry that he has met Voldemort back when he was known as Tom Riddle; that Tom didn't go by the name Voldemort back then. He is linking Voldemort's current title to his birth title. Ex: My mother's name is Mary Smith or as she is more commonly called Mama Mary. Ex: Lilies, or as they are scientifically named Lilium, are very beautiful. Ex: Tea, or as I like to call it, leaf juice, is my favorite drink.

I normally use the word "sheet" to indicate "paper". However, does it also refer to blankets as well? If so, could you give me some examples?

Sheets don't refer to blankets, they refer to the very thin layers of cloth that cover the mattress of a bed. They're usually categorize into two types: flat sheets, which just lay naturally on the bed, and fitted sheets, which have an elastic band all around to allow them to adhere to the shape of the mattress better. Ex: Can you change the sheets? There are some fresh sheets in the linen closet. Ex: I spilled coffee on my sheets, so I have to wash them.

What's "ride along" mean and when's it used?

"Ride along" means to "ride with," or in this context, "drive with" on the road. It can be used when you're driving with someone somewhere. Or, if you're riding your bike or driving next to or near a place, like a road or a river, then "ride along" can also be used. Ex: Do you wanna ride along to the mall? Ex: I rode along the river last night. => ride one's bike next to the river

What does “ace something” mean here?

In this context, "aced" means that a student got the letter grade "A" in a class, or multiple classes. For example, this sentence means she got all "As" in her probability and statistics classes. Ex: I aced all of my classes this semester, so I am proud of myself. Ex: She aced all of her science classes, that's why she's so good at science.