What is "sec"?

Why do we put "you" next to "don't", not just "don't take the easy way out"?

"You" is added here for emphasis addressing the other person. "Don't take the easy way out" can also be said as is. Adding "don't" after "you" is also part of a dialect that is spoken in the Southern part of the United States. People in the South of the U.S. sometimes put "you" after "don't". It is not correct grammar, but it is how some people who live there talk.

I often see a subject and a verb is omitted in verbal English, is it common?

Yes, that's correct. In casual conversation, it's okay to omit words in this way. The most commonly omitted words are articles (an, the), subjects, and verbs connected to these subjects. You can think of "but why use the word "fired"?" to mean, "but why do people use the word "fired"?" Ex: Why cry for me? I'm not sad at all. (Why are you crying for me?) Ex: Sit down over there. (You can sit down over there.)

Can I use "and" instead of "but"? What's different between "but" and "and"?

The conjunctions "and" and "but" can be used to join two statements. "And" is used to show additional information and is used when the statements are similar. Ex: I called my mom yesterday and we talked for hours. "But" is used to connect statements that express opposite ideas. Ex: I also called my brother, but he wasn't at home. In this case since the "party" is dissimilar to giving "money" I think but is the better choice.

Is it grammatically correct to use "on there" in this sentence? Or, is "on there" used correctly in terms of meaning in this context?

Yes, it is grammatically correct to use "on there" in this sentence. "On there" is in reference to on top of the pulley system that they are creating. It is common to use "there" or "that" in reference to parts of a sentence that can be omitted.

Why does "Moses" suddenly appear here? Could you explain the phrase "play the part like Moses"?

This is a bit of a word pun. Moses is known for parting the Red Sea, and creating a divide in the middle that allowed him to cross safely. The lyrics say "play the part like Moses," with the "part" here meaning "role." The lyrics do not mean the same "part" that means "divide" (the Red Sea), but it has still used been used here due to the same pronunciation. Ex: Moses parted the red sea. Ex: The actor played his part very well.