Sorry, sdRohan, but I was not willing to go that far on this system - just yet.
HOWEVER, with your thought process in mind I decided to go through those steps part way and individually disable startup services one-at-a-time via TM, reboot and see what happens.
And it paid off. To my surprise, it was UltraMon. As seen here, it was last updated last November and has been working fine with Start10 since then which is why I did not suspect it. Only after this last round of Windows Updates did the problem appear.
Now I'm an electronics technician, not a programmer so I am out of my element here but it seems clear to me one of the recent Windows Updates caused a conflict in the way UltraMon interacts with the W10 NVIDIA drivers multi-monitor setup with my graphics card, which I suspect, is similar to how Start10 deals with multi-monitor setups.
UltraMon did not load until late in the boot process, after Start10. I assume that is why Start10 appeared as expected, but then disappeared near the end of the boot process.
So I have removed UltraMon from the Startup folder. I logged out and back in, rebooted, and did a full "cold" shutdown and restart and Start10 started and remained in place each time. As a final test, I just started up UltraMon manually, at least 30 minutes after the last boot. Immediately after the UltraMon System Tray icon appeared, the Start10 button vanished.
So I don't know if UltraMon is calling some routine it should not be calling, or if Start10 has some vulnerability that should not so easily be exploited. Again, I am not a programmer. But it might be something you might want to play with.
For now, I will run without UltraMon and hope that in the near future, it will be sorted out so I can run both again.
@witafrog - at this point it might good if you started your own thread to deal with your problem as clearly you issue is different than mine.