Microsoft 365 - adding a domain where admin was unknown or lost password

There was an issue adding a domain to our Microsoft365 non-profit tenant - that domain had been added (probably by mistake) to another tenant thedomainonmicrosoft.com

The admin for that domain was not known, or the people involved had forgotten, and also forgotten the password for the admin login.

Since I was a global admin for the target tenant, first tried adding, then tried "domain takeover" which did not work, powershell - 'Connect-MsolService' is not recognized as the name of a cmdlet - Stack Overflow

then contacted Microsoft support as mentioned in admin.microsoft.com domain console. 

I had set my preferred method of contact as email, but the request was passed on to Microsoft's Data Protection team (who guard against scams and illegal takeovers) who had to call me to verify etc. Then they emailed the admin of the domain to verify, and they emailed back saying they have lost the password, and that this domain move should be done. But the Data Protection team does not do domain removals - they called and verified the admin contact, and then reset the password for the admin contact. Once that was done, they themselves could remove the domain from the onmicrosoft.com tenant and I could add it to the non-profit tenant. 

The whole process took a week, because of the multiple verification requirements, and because the various people were available only during some hours of the day. But the experience was not unpleasant, since the support people took the time to understand the issue and resolve it without simply ignoring it - as it happens sometimes with issues raised with Google.