Unless you have alternative E-Mails or Phone Numbers set up in the Admin Account (which I dont believe since you described your problem detailed) you have no choice but to call Microsoft. They can verify the Corp Identity but you need following Documents prepared

Proof of domain ownership for your Office 365 tenant (such as DNS configuration details).

Account information related to your Office 365 or Azure subscription (e.g., subscription ID, registered billing info).

Any previous billing statements or email records related to the account, to help verify ownership.