Will server to server migration work cross-domain/cross-active directory?

Yes I have looked at the tooling and it's a non-starter for many reasons, expense not the least of them.  But I don't want to get into ripping the commercial tooling apart.  I'll just say that I've yet to see any commercial Exchange add-on program that I cannot duplicate with a Copilot-written PowerShell script running in Exchange Management Shell.  As you are aware the analysis programs and reports that come with Exchange are nonexistent.  So if you want to produce pretty graphs of what's going on in your email environment to wow the C-Suite, you better learn how to write PowerShell.  And as it turns out, after Microsoft came out with Copilot, you don't need to do that you just need to learn how to talk to Copilot and tell it what you want it to script and with a few hours effort on your part, you will get usable PowerShell scripts out of it to do just that.

Fundamentally I'm a lazy admin.  If I can have the computer do the work, I do it.  If I can have the computer write the tool I need to have the computer do the work, that's even better.  And so far I've been able to do that.  I can coax Copilot to write me a batch migration script that will copy a group of mailboxes from one server to the other.  The actual migration of mailboxes is the easy part.

The problem is to the world for the last 3 decades the only legitimate email addresses for this org are the wonkulatinggronkulator.com ones. So, in any phased move, there will be legitimate email addresses in wonkulatinggronkulator.com on BOTH the old and the new Exchange servers, with the old server in a separate forest from the new server.  In other words,  BOTH servers believe they are authoritative for wonkulatinggronkulator.com  

In a  "same org, different servers" AD, all in one forest, you can tell 1 server "you are the public face" and queries from Outlook, from Outlook Web Access, and so on, will all go through 1 server which will proxy them for the other server - the public server knows if the mailbox is on itself, or on the other server, and if it's on the other server, it will get the emails from the other server to present them to the user.

But in this case, that's not, apparently, going to work.  At least not any way that I can see to do it.