Multi-Geo & Multi-Domain Setup

JillArmour 

I know little outside of Dynamics and Power Platform but I'm willing to give this a go in hopes that the others might have more insight.

1. If the US tenant you want to use as the primary has not been established, establish that first.

2. Leverage tools from AvePoint or BitTitan to help migrate mailboxes and other data

3. Billing should be consolidated by virtue of having all licenses under a single tenant.

4. I don't know much about the domain setup part but I'm almost certain I've seen tenants where some users were mailto:email address removed for privacy reasonsand others were mailto:email address removed for privacy reasonsso it should support having multiple domains.

If you are a Microsoft partner, you can use some advisory hours to open a TPD (Technical Presales/Deployment) request and that should get you a conversation with someone knowledgeable enough to help you plan this out but execution will still be up to you.  

If you are not a partner, you may wish to engage with one who has proficiency in these areas but also these are the partner forums, not for end-user questions. 😉

If you have a relationship with an Indirect Provider (disti) they may also be able to provide some level of support or guidance.