I don't think he's asking about migrating data so much as asking about doing testing prior to the cutover. Unfortunately, I have not seen a special/hidden domain for Google Workspaces before where you could do as you are asking Blue House. However, I would trust that as long as you set up the MX record properly during the transition, email will flow properly with defaults on Google Workspaces.
There also is NO WAY to migrate one mailbox at a time as you are hoping. The MX record repoints where all email for the domain is delivered, so without some temporary domain on the Google Workspaces accounts that has proper MX records for itself, there's no way to test those mailboxes for mail flow.
If you're really worried, I'd suggest exactly that. Buy a different domain for super cheap, add it to Google Workspaces with all the defaults and the MX records, then test whatever you want to test about mail flow with the temporary domain.
As for the actual MX record change, I've done this loads of times and generally it just works properly if you do the following:
1) change all the existing MX records to a really short TTL ahead of time: 30 minutes or less if you can, shortest available if you can't get that quick
2) copy/paste what Google or whatever service you're using gives you rather than typing it in yourself
3) check to see if the DNS service uses any funky notation (some servers make you put the . at the end of records for instance) ahead of time
Google and Microsoft do an excellent job of giving you all the right info for DNS moves for their services. I wouldn't sweat it too much. As long as you make sure your clients understand there will be down time and you do it when that can be acceptable (even if there isn't actually any down time because you've rigged it that way or something), then failing back in the case of catastrophic failure isn't a problem either.