Outlook send mail behavior with shared mailboxes

Hi there, 

I'm looking for some guidance with some send mail behavior I'm seeing in Outlook.  

First a little backstory:

My company, Fabrikam Pharma, was recently acquired by Contoso LTC. The fabrikam.com domain was moved into the Contoso LTC Office 365 tenant.

Before the domain migration, some Fabrikam users were already given contosoltc.com user accounts (the merger was not legalized or made public).  After the migration, such users had their fabrikam.com emails merged into their contosoltc.com mailbox. Their Fabrikam addresses were made aliases of their contsoltc.com account.

Most people loved having one mailbox, but a few executives didn’t.  They are adamant about separating the two accounts, so we turned their Fabrikam aliases into shared mailboxes.

So now they toggle between the two addresses when sending a new message. The problem we’re having now is the primary domain, contosoltc.com, is always the default address, even if they’re creating a message or replying from their fabrikam.com mailbox.

Additionally, any Fabrikam messages/replies appear only in the Sent Items of the Contoso mailbox.

Here are a few things I’ve tried:

1. Changing the https://docs.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/sent-mail-is-not-saved registry setting.
   It works nicely for keeping sent items separate but does not address the problem of the from field defaulting to the Contoso address when replying/sending from the Fabrikam mailbox.
2. “Copy items sent as this mailbox” option in the Shared Mailbox settings.
   That option does exactly what it says---Copies the message to Sent Items of the shared Fabrikam mailbox, but it still exists in the Contoso mailbox.  The execs want those separate too.
3. Adding the shared mailbox like I would a full account and making it the default. Inspired by https://answers.microsoft.com/en-us/outlook_com/forum/all/setting-the-default-send-from-e-address-on-shared/b06039be-c4dc-4bb3-8b9c-d31315c84bf8?page=1.
   It works great until Outlook closes. When Outlook is opened later, Outbound messages fail with the error “Outlook data file cannot be accessed.” It’s like the profile doesn’t load properly. I cleared the error by restarting the computer, but that’s not a workable solution. This hack didn’t work well for https://answers.microsoft.com/en-us/outlook_com/forum/all/office365-shared-mailbox-occasional-0x8004010f-in/3af46151-37c9-476b-843b-ebe02873b9de as well.

The behavior I’ve described above only impacts Windows users who are heavy users of the Outlook desktop app. Users of Outlook for Mac are not experiencing these issues at all.

I’m starting to think the only way to get these folks the mail behavior they want is to convert their shared mailboxes back to a user account and give them another Office 365 license.  Is that really the best way to go about it? What other problems could arise from having two licensed user accounts in one tenant?

Many thanks!