License, copyright, family court?

(little info in advance, I'm under 18)

I want to open a small shop where I want to sell websites and minecraft plugins (I wrote myself), now I want to convince the potential customers that a free website is offered so that the effort is "worth it", I have on the Internet Searched for free websites and found templates. Their terms say that it is a template and that I should also use it as a template. Here are the terms https://www.free-css.com/contact-us

Now summarized again:
I'm under 18, I want to offer a small shop with a colleague where I want to sell plugins and websites, then I thought about doing a free service where customers can look at it, and now one of my questions is whether I can may give the websites for resale, so resale not literally, but that these template websites are mine and I can offer them as a free version as a test for my service. (Change everything to mean or change the data given by the customer)

Family court?

Do I have to register a small business for the small shop that is guaranteed to make less than 450 euro a month?

So do I need a small business and have to go to the family court with a business plan? Because for a little bit that would be a lot of work. Well, of course, I want to achieve something, and if we generate income, then of course I'll report that.

Summaries of all questions:

So can I rewrite the free website templates as my templates for my customers, as these are free templates available for download, or do I have to specify the owner and not give them as a test version for my customers. Would just rewrite the templates so that it is a suitable text for my customers as an example.

And do I have to register a small business when starting a small shop with currently 0 euro income? If so, do I have to register before the website even goes online?

Because I'm under 18, and then have to go to family court with a business plan I made. Then they have to approve it etc.

Before you even think about the shop and trade, you should first ask the makers of Minecraft because otherwise it can be very expensive for you / you.

So I can't program plugins and sell them to customers? Because the customers have to get a hosting provider themselves, on which they upload the plugins ^^

https://account.mojang.com/....com/terms

I'm not pretending that the plugins are part of the official game. Because, as an example, it is just a lobby plugin for their server. And I can also put a book in-game in the inventory when it starts, where it says that it was not approved by Mojang ^^

Spigot wasn't approved by Mojang either

As I said, deal with the game makers because you've come to the right place

But not every plugin developer, writes Mojang, because they couldn't work that moderately there. Well thank you