1.
If they cannot help you, I don't know.
2.
Contact the shop / seller where you purchased your Mac, see -> Keep your receipt after you buy an Apple device -> Keep your receipt after you buy an Apple device – Apple Support (UK)
3.
You are sign in here . . . with your Apple ID & Password.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
Use the same Apple ID & Password that you use to sign in here?
See -> Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support