I don't understand the usage of "I think" in a grammar standpoint that's in the middle of this sentence. Because "will appear" comes after this, it doesn't look right to me. Or is it only okay when speaking?

Is it possible to say "reprogram" for living beings as well?

No, you couldn't "reprogram" a living being since living beings aren't built with programs. However, you could use it in a metaphorical sense since, as humans, we can change the way we think or do things. Sometimes people call that "reprogramming." Ex: We can reprogram the way we think about ourselves if we intentionally think positive thoughts. Ex: Unfortunately, our dog isn't a robot. Otherwise, we'd reprogram him not to bark. The most we can do is train him.

What does "your highness" mean?

Your highness is used to address someone in the Royal family.

What does "go out for" mean in this sentence?

In this sentence, "go out for" means to try to achieve/get a position. This character is trying out for the position to be Keeper against Ron Weasley. This is not a typical phrase used in American English. In the U.S.A, "trying out" is used more often. An example sentence of "go out for" would be: She is going out for drummer in that new band. An American would probably say: She is trying out for drummer in that new band. Either is fine to use.

What does "one" from "one lasagna for every year old you are" mean? Can you give me some example sentences using "one" like this?

You will notice that earlier in the video, Garfield receives nine lasagnas from his owner. This number represents his age, which is why his owner says that he has cooked "one lasagna for every year old you are." Although the ending of the sentence is slightly awkward, you can use this grammar pattern in similar ways. Ex: For your birthday cake, you should light one candle for every year you've been alive. Ex: One present for each member of the family.

What does "I'm not feeling it" mean? I'm asking because this sentence seems like used differently from literal meaning.

The expression "I'm not feeling it" in this case expresses the same meaning as to not feel like doing something. So although "I'm not feeling it" sounds like a very casual expression with a different meaning, the meaning can be taken literally. Ex: A: Do you want to go to the beach today? B: I'm not feeling it. Ex: I was going to play guitar but now I'm not feeling it.