Well, it was a real nailbiter. Dban finished wiping my drive and it was time to insert the Win 7 OEM discs. Would they work? If not, I’d have hours more of searching, hoping to find a solution. But it’s working. So providing my backup discs are okay (I have two sets), I should be back in full operation by Tuesday!
Yay. Knowledge is good.
To make sense of this, see here: https://mclark.wordpress.com/2018/10/08/baffling-windows-10-7-upgrade-downgrade/