All attempts I have made to use migration assistant to move the old account from Time Machine have failed, even when I have created an account of the same name as the old account on a partition with the same name as the failed hard disk. It seems like the refusal to copy the long-ago moved account is written into the Time Machine backup, and there is no way to trick it into doing so by the way you set up the new machine.
Again, you cannot migrate a user account over the top of an existing account of the same name. Not possible. Won't work. Do not attempt.
Either run the migration from Setup Assistant or create a brand new user account when you set up the Mac, then use Migration assistant later.
I imagine that if Time Machine backed up a user folder on a non-startup drive, there is no way to use Migration Assistant to migrate that user.
I thought you had moved the account on the current setup and wanted to migrate it. Again, to migrate you would have to move the home folder back into the startup drive /Users folder. There is no way to do that in the TM backup. It must be done on the actual account before backing up.
So, there isn't a way to do what you want. Best you can do is "Browse other Backup Disks" in Time Machine (option-click menu) and restore individual files from the backup.
I don’t understand what he is saying about ‘broke the fusion drive in two’.
If you have a Mac with an SSD and an HDD, then it was supposed to be a Fusion drive where the two are joined and appear as one.
If they appear as two separate drives then you broke the Fusion drive apart, somehow. With them joined as a Fusion drive, there is no need to separate you home folder onto the HDD. The OS does that for you within the Fusion drive automatically.