Microsoft 365 administrator lockout

Disclaimer: Since you described your issue in English, I will also provide my response in English, even though the support site you are using is based in nb-no. If you prefer a Norwegian response, please feel free to let me know.
Dear @Rune Stoknes,

I completely understand how stressful and frustrating this situation is, especially since it affects your wife's business email and her service has already been cancelled. Dealing with long phone queues when a critical business operation is down is incredibly draining.

I can provide immediate clarity on the custom domain concern and give you an alternative path to get this resolved without waiting on the phone line for hours.

Will the custom domain be locked?

Yes. If you try to register a completely new Microsoft 365 tenant right now, you will not be able to add her custom domain name to it. Microsoft binds a custom domain strictly to one tenant at a time. Because the domain is still linked to her original, locked-out tenant, the new system will throw an error stating the domain is already in use. To safely keep her email address, we must restore access to the original tenant rather than starting fresh.

Since reaching phone support has been unsuccessful, you can use an official and highly effective workaround within the community: Create a temporary trial tenant solely to open a support ticket on behalf of the locked-out account.

You can sign up for a new Microsoft 365 tenant by creating a trial subscription here: Microsoft 365 Business Plans and Pricing | Microsoft 365  

Once that new tenant is created, you’ll be able to access the Microsoft 365 admin center and submit a support ticket directly from there. In the ticket, explain that you’re requesting assistance from the Data Protection Team for your original tenant, due to being locked out as the only admin with MFA issues. This method often helps bypass the automated loops because you’re able to submit the request from an authenticated admin center, even if it’s under a temporary tenant.

Important Note: Because this involves security and account ownership, the Data Protection Team will perform strict identity and domain ownership verification (such as confirming billing details or asking you to add a temporary DNS record to your domain hosting) before resetting the MFA. This process can take several business days as they handle high security verification volumes, but it is the definitive way to safely regain control of the business domain and update the expired card. 

Once the primary tenant is recovered and the billing is fixed, you can simply go into the trial tenant's billing section and cancel the trial to ensure you are never charged for it.

Also, just to set expectations clearly and transparently, this is a user‑to‑user support forum, so I don’t have the ability to intervene directly or unlock accounts. I can only suggest additional options based on my experience and what has worked for others in the community.

I hope this helps you and your wife get the ball rolling on recovering her business email quickly!