After installation of KB4056892 boot failure, after roll-back error 0x800f0845

tried to use its Tools menu but got taken to ms-settings:magnifier (or whatever it is--nothing obvious like that) which of course for you would require a completely functional explorer.exe.  

Found this (obscure syntax):   ms-settings:easeofaccess-magnifier

Actually I almost had that but used a slash instead of a dash for a subtopic separator.   Good to know.  Guess they are giving up on the pretense of calling these things URI (which IMO would require the // following the colon at least--otherwise it just parses like a relative drive specification).

If you want to try starting that without using the Magnifier window's Tools button you could use the following syntax from Task Manager's File New,  Command Prompt's or PowerShell's command lines:

explorer.exe  ms-settings:easeofaccess-magnifier

However, since it is now looking that your problem is going to involve the fully functional availability of explorer.exe that may not accomplish its normal function in your current state.

FYI

Robert

EDIT: Tried low resolution mode and seems to be the same result (only larger mouse pointer in low-res). I'm going to try letting it run a bit as described below to see if anything different happens.

Could there be a third display involved?  I have seen this before--windows get left on a screen that is no longer detected but was once primary and it stays primary until I switch it.   The problem with that scenario for explaining your case is that you can see the Project dialog and that means you are on the primary screen.  Also, with Low res I don't think you are allowed to have multiple screens.  In any case try pressing  Win+Shift+CursorRight  to "throw" a window over, especially if Narrator has identified it for you after pressing Alt-Tab.  Note that Alt-Tab lists all our windows regardless of their states, so it is a more complete list than Win-Tab but it is a less functional display than Win-Tab.  Also, when explorer.exe is stopped Alt-Tab can still be used but then it shows only the task icons and navigation is only available via Alt-Tab and Alt-Shift-Tab.  I think if we could figure out an easy way to kill explorer.exe with Narrator's help you might be able to start seeing something.  Unfortunately usually killing explorer.exe means that it is just restarted again.   Task Manager can do it but that might be too complex a task to try to do blindly even with Narrator's help.

 an easy way to kill explorer.exe with Narrator's help

Get into Administrator: Windows PowerShell and type and Enter this:

(Get-Process explorer).Id |% {cmd /c taskkill /PID $_ /F}

The taskkill /F switch works where PowerShell's Stop-Process -Force does not.  Thanks for making me find this.

Then note the difference.  All explorer generated stuff is removed from the display and stays removed.  More significantly Alt-Tab now shows only icons.  Surprisingly some modern app windows are still functional!  Who knew?  To get explorer restarted just do it conventionally (e.g. with Task Manager) or in PowerShell  Start-Process  explorer   works.

Good luck

Robert

Please use Narrator to keep control even though you can't see what is going on anywhere.   I'm mystified about that.  I was hoping you would be able to see more with Low res.  Note that as soon as any Accessibility program is running you would have something to show in an Alt-Tab list.  So, it is surprising that you can't see either Task Manager or Narrator on the screen.   We can ignore the state of explorer.exe in that case (or then learn to start focusing on its problems and try to figure out how to work around them).

Another one that would change the video to be viewed is Mangifier in its Fullscreen mode with its Reversed video option.  OMG.  They have "modernized" that too?  I  just got Magnifier started conventionally with Win++, got into its Views menu and switched to FullScreen, and then tried to use its Tools menu but got taken to ms-settings:magnifier (or whatever it is--nothing obvious like that) which of course for you would require a completely functional explorer.exe.  

<voice actor="Dorothy" >  Toto, this isn't Kansas, is it?  </voice>

Another quick update. I had to run some errands and just left it at the blank screen. When I returned, the typical beach/cavern/blue sky Win 10 default login image showed up on the screen with the selected user account and the spinning circle of dots which seems to indicate the user profile is still trying to load. However, it never finishes. I can get the narrator menu to pop up on the screen during this but no sounds are being generated (yes, speakers connected and powered on). And I can't select/click on any of the options. So the menu appears to be inoperable for some reason. Alt-Tab shows (2) selections, "Windows sign-in" and another selection that has no label of any sort. So something is definitely locking it up upon initial user account loading. If I only I could get to a minimalist boot, I might be able to do some clean-up on start-up processes and such.

At this point, my only options I can see are a Win10 PC Refresh or Reset (or worst case, complete new install). Unless there are any other suggestions on how to boot with limited system services/processes. On a side note, both monitors I have connected (1 DVI, 1 VGA) are mirroring the same display (i.e. beach/cavern/blue sky) so I don't think it's necessarily an issue with an extended/hidden desktop at this point. <sigh>

 If I only I could get to a minimalist boot, I might be able to do some clean-up on start-up processes and such.

That's what the boot menu idea could do but you also were on that tack via the Power button's Shift-Restart to get into Advanced Boot Options.