Your TechTool Pro 6 is antiquated and may be unsafe to use, but IF (and that is a big IF) it has an option to create a ProToGo drive you might try using that to create a bootable thumb drive to see if your granddaughter's iMac. ProToGo is a stripped down version of macOS and might boot where the full macOS would fail and it will run the TechTool analytical routines. Alternatively you can create a bootable installer thumbdrive of almost any macOS following these instructions. It is the equivalent of a recovery drive on a thumbdrive complete with Disk Utility.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein