What does 'sort of' mean?

Can I just say "for free" instead of "free of charge" here? by the way is something omitted before "free of charge" in the sentence?

Yes, you can use both phrases interchangeably in most cases. Nothing is omitted before "free of charge" in the example, but it may sound like it since it's a phrase that modifies other phrases and isn't introduced by any parts of the sentence, but this is because "free" is used as an adverb and the whole phrase is adverbial in nature. Ex: You can use the spa free of charge with your gym membership. Ex: The hotel lets you take one towel free of charge because people steal them anyway.

What's the difference between "hold on", "hang on" and "Just a minute/second"? Are they always interchangeable?

These expressions have similar meanings and are usually interchangeable in situations when you'd like to tell someone to wait a moment. "Hold on" and "hang on" also have additional definitions, however, as they are used to also mean "stop" or "wait," and also as encouragement to persevere or endure in difficult situations. In this video, "hold on" is closer to the last definition, meaning for Bill to endure or persevere until he can get help. Ex: Just a moment, I will help you as quickly as I can. Ex: Hold on, you're driving too fast! Ex: Hang on for just a little while longer, the exam season will be over soon! Then you can finally have a break.

What's the difference between "grow" and "harvest" here?

To "grow" is to plant seeds and then help to increasing in size and change by watering or feeding a plant. To "harvest" a plant is to collect or gather plants by taking them out of the ground. Ex: I am growing roses in my garden. Have a look, they're so pretty! Ex: He just harvested these apples. They're fresh from the orchard.

What does “get something out” mean?

"Get it out" here means "talk about it". "Get it out" is used in this way when someone wants to talk about something that is bothering them, and they will feel better if they tell someone about it. Ex: Thank you for listening to my story, I had to get it out. Ex: I have something I need to get out, can I tell you about it?

What does "batch" mean here? Is this word normally used in the lab?

A "batch" is a quantity of something that was produced at the same time. "Batch" is a word that could be used in a lab. Ex: I baked a batch of cookies today. Ex: We are going to test this batch of samples in the lab this week.