Buying add-on for Xbox affecting another account

Yea you should be able to turn off account share in settings.

Yes content sharing is a feature provided the account doing the sharing is set as your home console (which happens if it was the first account on the console during setup.)

I dont know if it's a full fix, but you can remove the tag of 'home xbox' in the settings, this may stop the ability to share content.

See here for more. https://beta.support.xbox.com/help/hardware-network/console/my-home-xbox

Like buying a DLC or something? Then no because purchases are tied to the account that made them.

I had a sign in error so I removed the account as recommended. Upon readding the account, it says "(user) is here already. Another user on this device uses this Microsoft account, so you can't add it here."

There are no accounts whatsoever under the sign in menu or delete account menu. This is the only Microsoft account I have ever had. So i now cannot delete or add the account??

https://beta.support.xbox.com/xbox-live-status. There is a service alert that might be affecting this.

Correct. You can create a completely new Xbox account and his Fortnite data will be saved. When he logs onto fornite he needs to sign in with his epic email and password. That's up to you. And no it will not affect the information. If you add another account to that xbox and you make it you're home Xbox you're sharing your gold with him. Therefore you don't have to pay for both accounts.