Further update:
To rule out corruption of system files, I used Microsoft's DISM tool to compare and re-download any system files that don't match their proper checksums (DISM.exe /Online /Cleanup-image /Restorehealth) and then an SFC verfication and restore (sfc /scannow), so all system files have been verified as correct and unmodified, and all Windows Updates are also installed.
Windows Defender remains disabled during testing.
