Hello, and thank you for reaching out. You can use these textures however you would like, even in your purchasable game! We would greatly appreciate a mention in the credits but it is not required.
yes, the textures are seamless. some of the textures such as the "walls" are only seamless horizontally but most are fully seamless.
We are excited to hear that you are making a game with these assets! We look forward to seeing what you create and would be honored to give your project a shout-out when it's ready! Please let us know if there are any other questions!
(Also, your English is great!)