Thanks very much for your response and advice -- I had run the dism /restorehealth earlier (but without the /startcomponentcleanup switch) on the laptop which was experiencing the endless CHKDSK, that never budged beyond 10% even though I'd let it run for several hours before I had made the initial post. That symptom was unique to the 1809 machine, which afterward had crashed with a BSOD followed by some Dell-flavored recovery process that was wanting to reinstall a completely clean image (i.e., return to an out-of-box state) -- I chose instead to toggle the power-button icon that was offered and rebooted to a normal desktop, at which point I ran through all of those commands as you outlined in an administrator-enabled Powershell window, but specified the /X switch in the CHKDSK command to dismount the volume. This seems to have completely resolved the WU issue it was experiencing, and all seems back to normal.
Not so for the 1803 laptop (a Toshiba with space-constrained SSD, so unlikely to experience a drive failure); although I've run the various Windows troubleshooters and had them "fix" the issue(s) found and then through the same process you outlined that worked with the 1809 laptop, that 0x80070643 error continues when WU runs. In looking over the Event Viewer details, all but one of the most recent failure is associated with the standard KB for Windows Defender updates (KB2267602), but the latest includes an additional entry for the latest security update (KB4023057, the 2018-11 update) -- MS indicates this update is available through WU, but not independently through the update catalog.
Further thoughts?