As per your Query: One of the big hurdles will be the fact that we're using the same externaldomain.com UPN on-premises across both Exchange forests. Maybe we need to take a calculated risk of this?
You’re going to do two separate things: remove Azure AD Connect and migrate Active Directory accounts from the old forest to the new. Firstly, execute: “Set-MsolDirSyncEnabled –EnableDirSync $false”. This will disconnect the relationship between Azure AD and on-premises AD. The objects in Azure AD will change from being synched to being Cloud-Only. The user IDs and passwords are all the same and users can log on to email, etc., and send and receive just as they had been doing up until now.
This enables you to focus on your Active Directory transition project without the worry that in 20 minutes you’ll have users breathing down your neck and clamoring to be in their Office 365 resources.
Your options are (at least) three:
ADMT from Microsoft: https://www.microsoft.com/en-us/download/details.aspx?id=19188
Migration Manager from Quest: https://www.quest.com/products/migration-manager-for-active-directory/
Active Directory Pro from Binary Tree: https://www.binarytree.com/products/active-directory-pro/
Azure AD Connect has come a long way over the years. The applicable feature in this case is SMTP Matching: https://support.microsoft.com/en-us/help/2641663/use-smtp-matching-to-match-on-premises-user-account...
. The key factor in the migration process is to make sure the migrated source object in Active Directory and the target object in Azure AD have the same UPN and SMTP addresses. If the object you’re sending to Azure AD has the same UPN and/or SMTP address, the account will change from being cloud-only to being synchronized with the new Active Directory.
So I think since you have same Externaldomain.com UPN this should work. I would also suggest that you do a LAB first with a minimal set of AD users and ADMT tool. Also you can use the trial version of E3 or E5 for your LAB creation.
However the only difference in the above article LAB is the UPN's are different for Forest A and Forest C users.