Questions about migrating Microsoft 365 tenants, between non Microsoft "connected" geo-regions?

I am planning on migrating an existing Microsoft 365 tenant, from a Microsoft geographic region that doesn't have a direct backend migration path to the geographic region where my new tenant needs to reside.  The key requirement is that at the time of cutover, the new tenant will need to have the same domain name associated with it that the existing tenant currently does, with as little downtime as possible.
So I understand that I am going to have to stand up the new tenant, perform a pre-cutover migration of ALL data from the current tenant to the new tenant, but have a few questions about the cutover phase.

(NOTE: I've staged the new tenant with a temp placeholder domain, so that I can create all the new user accounts, shared mailboxes, MS Teams, etc. that are going to have their data migrated from the current tenant.)

1.   When I disconnect and remove my existing Internet domain from the current M365 tenant, leaving the old tenant using only the company.onmicrosoft.com domain as its default, how long does it usually take on the backend before I can then re-add that domain to the new tenant and make it the default domain?

2.  If the autodiscover CNAME record for my migrated Internet domain remains as is in the public DNS zone record, once I make it live in my new tenant, should my users' Outlook clients, both mobile and desktop, automatically reconfigure themselves to be pointing at their mailboxes on the new tenant?  I keep the TTL on my zone files very low, so they refresh quickly.

3.  When the user's Outlook client(s) gets redirected to their Exchange mailbox on the new tenant, how can I be sure that Outlook won't see all their migrated emails as "new" and prevent the clients from syncing a duplicate copy of everything it finds?