Which direction do you want the QS8s facing - "out from the fireplace" or "out from the white wall" ?
I understand you don't want to mess up the fireplace but my first thought when I saw the DeWalt (drill ? screwdriver ?) in the second picture was...
"if drilling into the fireplace is not an option then maybe you need a bigger drill"
Rather than FMB, how about making wooden 90 degree brackets painted to match the wall plus the standard wall mounting/hanging plate ?
EDIT - interestingly enough the QS8 stands still show up on Amazon, and if you click on "sellers" it says "Axiom"
https://www.amazon.com/Metal-Stands-Axiom-Quadpolar-Speakers/dp/B000FEGIUM