Reply All from Shared Mailbox removes Sender Address and Duplicates Shared Mailbox Address

It's optional, but need to be configured via PowerShell. Remove the permissions and re-add them with Automapping turned off:

Remove-MailboxPermission mailbox -User delegate -AccessRights FullAccess

Add-MailboxPermission mailbox -User delegate -AccessRights FullAccess -Automapping $false

The mailbox will not automatically appear in Outlook then. To add it (as additional account), use the File menu -> Add Account -> Enter the shared mailbox email address and dont enter password. When prompted for credentials, enter the UPN/password of the delegate user.