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Basically this.
Its all now registered to his account, you'll need to logout of his and login to yours.
What you could do in the interim (and @CYOA to confirm this will work) as I guess you odnt want ot open up your iMac...
Buy an external SSD and Caddy, instal macOS to SSD, migrate time machine to new SSD, use as boot drive.I'm slightly confused as what has actually been done - the old HDD failed? You fitted a new HDD?
This might explain the photo and time machine issues?