What does "plop down" mean?

Is "who is" omitted between "nobody else" and "as good as"? Like this, "nobody else (who is) as good as you"

No, it's not omitted. But it could be added! They would mean the same thing here. I will say that sometimes adding "who is" makes the sentence longer than it needs to be. Ex: I can't find anybody as funny as you = I can't find anybody who is as funny as you Ex: She can't find anybody as good at chess as you are.

I assume the word "mediocre" is a synonym of "ordinary." However, if I replace it with "mediocre," would it imply a negative nuance?

You're right. They are synonyms, but they're not interchangeable here because mediocre does carry a negative nuance. "Mediocre" implies something of not great quality, whereas something can be "ordinary" and still be "good," or better than average, depending on the context. Ex: It was an ordinary day. I went to work, had lunch, and came home. But It was still a good day. Ex: Today was a mediocre day. It wasn't amazing or terrible. Ex: This is an ordinary sandwich, so why do you like it so much? Ex: All the food I make is so mediocre. I get bored of it.

What's "be humbled" mean?

To "be humbled" means that you understand or become aware that you're not as important or special as you thought you were. You become less proud due to awe, inspiration, or admiration. It could also be because of gratitude or underserved advantage. Ex: I was humbled when I went to the museum and could appreciate the technological and social advantages we have today compared to the past! => gratitude Ex: I'm often humbled by the kindness of strangers.

How do you usually use "wait"? Is it different from "hold on" here?

"Wait" actually has a similar meaning to "hold on," "wait a moment," "just a moment," and "hang on." Ex: Hold on. What? You quit your job? Ex: Hold on. I'll be right back.

What does "off" mean here? Is the meaning differs in the case of saying "wheel her to the delivery"?

"Off" here means "away", which implies that the delivery area is in another part of the building. Therefore, "wheel her off to the delivery" has a different nuance than "wheel her to the delivery", which means the delivery could be anywhere.