There’s not much you can resolve. Apple highly restricts the ability to reuse an external ID already associated in any way with an existing iCloud account if anyone (including you) makes a new iCloud account.
You’re not sharing where exactly this long ago created account has iCloud.com email address associated. It could be parental, security backup, trusted recovery, primary Apple ID or simply what the system suggested for your iCloud when your brought (for instance) a Gmail or yahoo or Hotmail email for your Apple ID. Same confusion exists on how your other emails are used. Only Apple support can clearly see your account unless you document a ton of screen shots or share a screen with someone for about 15 methodical minutes to explore all nooks and corners of iCloud service setup.
Every iCloud account has the option have an iCloud mail address and it’s very common (or was in the past) for people to click through and opt in to an iCloud or .me or .mac email and then ignore that if an external email is their log in string to the account (instead of a phone number or iCloud email).
I would focus on how secure the account is now and avoid making a new account unless you understand in writing from Apple support clearly that you will be able to detach an external mail from the old account and exactly how you will use or attach it to a new iCloud account.
It’s far easier to just secure the account you have. No one can use or abuse that [email protected] if you have two factor security and a strong, unique password.
In summery, don’t try to resolve this. Secure your account if it mostly works. Work with Apple support on this, they are far better than we can ever be with securing your specific iCloud.
