Advice regarding 365 hybrid, acquisition with multi-forest.

I would be grateful for any advice on the best way to proceed in our scenario that I’ve outlined below. There seems to be multiple ways to resolve this issue but no clear guidance on what would be best practice.

Our setup

Original Setup

• Originally Single site firm, on premise exchange, one domain.
• Acquired a firm that was 365 only with their own tenant, with no exchange and using business essentials to synchronise local active directory to 365.

Our process so far

• setup a hybrid between our on-premise exchange 2016 and our own 365 tenant
• D. connect to synchronise our users with Azure A.D.
• Setup ADFS and single sign-on to authenticate 365 on our tenant
• setup trust between our active directory and the acquired firms active directory
• Use a third-party tool to move the mailboxes from acquired firms tenant to our tenant.
• Reconfigured A.D. connect to synchronise both forests to 365.

Current setup

• Acquired firm logon to our tenants using their own UPN authenticate on our ADFS server which authenticates their domain via the Forest trust.
• From 365 they are currently still sending out using their email address.

The problem

We have no intention at the moment of moving the mailboxes our mailbox into 365, we now have an issue with email routing, access to global address book, access to busy free between the different types of mailbox (one created directly on 365 and the ones that that are linked to our exchange)

Any email box that we are creating our exchange using the remote 365 creation procedure works fine as there are user already in our active directory, this means that in our on Prem GAL, we can access busy /free, they have the target address attribute in active directory set to onMicrosoft which allows for correct email routing.

The mailboxes from the firm we acquired that were brought over from their tenant do not appear in our active directory, and therefore our exchange has no idea of them. This means they do not appear in our global address book for on premise users, can’t access there busy free, and if we changed there email domain to the same as use then mail flow would not work.

The point of this long scenario is to understand what is the best way to get them in our local address book.

 The options

                Create contacts in our exchange, with both the public email address, and the external email   

              Address as onMicrosoft.

             Create users in our active directory, link them back up to the acquired firms 365 mailboxes by using the exchange GUID using  power shell command, then disabling the account in our active directory, and not allowing it to sync from our A.D. connect

Using a third-party tool to gal sync

Creating a resource forest, with the acquired firm users appearing in both active directory is matching them on the master exchange attribute.

What is not sure the exact way to go and appreciate some general advice on what way and what would be the best way to support this.