Update - I created a Defender CD, and it was able to start at boot & scan the Windows 10 laptops. No errors for either the start or the results. I used the 64-bit version. Note that neither of the laptops I scanned have the newer UEFI type of Bios.
I noticed, though, that on the Microsoft website from which we download the Defender Offline Scanner executable (which in turn, when executed, downloads the latest versions of the scanner & definitions) that the page shows a banner message "This page is for previous versions of Windows" ... and that when I went to the Windows 10 version of the page, no such download existed [so I went ahead and used the one for Windows 7/8/8.1]. The page for Windows 10's Defender mentions that if the locally installed Windows 10 Defender finds a file that it cannot delete, it will prompt the user to download the Offline Scanner for Windows 10's version of Defender --- it doesn't mention if that download creates an .iso file, a CD/DVD image, a bootable USB, or if it simply installs & is set to run in the pre-boot environment. If I find out anything more later on, I'll pass it on.