It seems that a "cumulative update" for W10 left my tablet in an unusable state (no input box for password or PIN). After much trial and error, I've just succeeded in a clean install of Windows 10. This is how I did it.

Don't turn off Secure Boot. The C: drive is encrypted (bitlocker) this way, and I understand (dimly!) that Windows 10 needs Secure Boot on at the time of installation. (Comments welcome)
You need:
- A USB memory stick with 32-bit installation media, usually via the Media Creation Tool
- A "4 port OTG adapter hub". This gives three standard USB ports, for memory stick, mouse, keyboard (plus power, which never seems to pass through) terminating in a single micro-USB plug for the tablet.
- A completely full battery

Now enter Setup:

Connect the adapter, USB stick, mouse & keyboard while powered-down.
From cold, power up the tablet, and immediately press the Volume-Up button repeatedly until the Setup screen appears.
On the Boot tab, I saw my USB stick offered as an option in the boot priority list. Tap the first item in the list to change it to the USB stick. Boot Manager drops to second place.
On the Exit tab, pick the option to save changes and exit.

As it restarts, you should see the USB stick LED (if it has one) flickering as files are loaded. Use the mouse and keyboard to navigate through the stages.

When you get to the choice of install location, you won't be able to "repair" or install over the C: drive due to the encryption. I chose (fingers crossed) to delete this partition entirely, and also following Recovery partitions. (I think I needed to restart and retrace as above to proceed. I did, anyway.).
I chose to pick the "Unallocated Space" as the destination, assuming (apparently correctly) that the installer would format as needed.

It chugged through all the stages, and after creating a basic account I now have W10 (1903) running on the tab. It's a very basic install - no touchscreen yet, nor wifi - will need to load the drivers from Dell. So far, so good, though. (I've have been just as happy if the original update - which was shown as some sort of extra option - had worked correctly, nevertheless.)

Worth noting that another contributor to these pages recommended instead holding the power button to force shutdown while the "juggling balls" are displayed. Do this three times, and (he says) the repair options appear, including the option to use a command window, and maybe reset Windows. That might be a better option if you can get it to work (I had tried, but I'd lost patience and was massacring partitions by the time I read his comment). But if you can get it to work, you may then be able to access the (encrypted) C: drive with Secure Boot still in effect. In my case, running the SC command may have been enough to configure the Credentials Manager service to Automatic (plausible solution to my original problem), and save me all the trouble above. Try that first!

Hope that's useful.

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