Can't setup paid Microsoft 365 subscription on native iOS/iPad OS apps--no Microsoft login popup

After spending HOURS on this, I think I may have diagnosed the issue. More testing needed, but in a nutshell:

1. Somehow I have two Microsoft accounts with the same email address: (1) my paid personal M365 account with custom domain; and (2) a business account with a Microsoft Fabric (free) subscription. I was clued on to this when I went to https://account.microsoft.com/account and after entering my username, a message popped up saying I had a personal and business account. Clicking the latter allowed me to log into a Microsoft 365 admin center which M365 Personal users should not be able to do. I can't seem to delete the business account using the Microsoft account deletion tool, so I'll have to contact support tomorrow. I was able to change the email address and remove my phone number on this business account which allowed the next step to work...
2. Microsoft's iOS setup directions appear to be wrong. Once I changed the email address on the 'business' account, I was able to setup M365 on my iPad by using the "Outlook.com" option. The "Microsoft Exchange" option still doesn't work for me. This is bizarre because I don't have an outlook.com account and ALL documentation says we're supposed to use the Microsoft Exchange connector. I don't know if "Outlook.com" connector is the right long-term solution, but it's working right now.

It's logical that two accounts with the same login username would cause the connectors to choke, but very odd that the Outlook.com connector would work but the Exchange connector would not. What a mess. Also, the three Microsoft M365 support people I connected with were abysmal and utterly witless, as seems to be par for the course with Microsoft.