Are there items (such as additional content, offers or currency) for sale that are used in the gameplay of your product?

Yes, it is possible to buy diamonds. Diamonds are used to buy gold and elixir.

Can your product be used without making any in-app purchases?

Yes, the game is not designed to take advantage of additional purchases, you can get diamonds by completing missions and achievements in the game.

Are in-app purchases refundable?

It is not possible

What payment platform is used to facilitate these purchases? Do you use Epic Games’ platform or a 3rd-party (including your own) payment system?

Only Epic Games platform.