uefi hardware diagnostics install failure stops booting

[Rogio1]

Elitebook 840 G2 was telling me to calibrate my battery every Tuesday.  Couldn't get the calibration to run until installing the HP Hardware Diagnostics UEFI.  Ran some of the other checks in the utility which all worked fine.  Then tried the Battery Calibration selection.  Followed the instructions and waited 6 hours for battery to drain.  Reconnected power and it self-started to windows so I couldn't do the 6-hour charge or 4-hour wait before powering up.
Assuming there was an issue with the installation of the utility, I reinstalled it (Didn't uninstall it, first).
When the machine restarted, It was stuck on the first (Language selection) screen for the HP Hardware Diagnostics setup.  The mouse cursor is visible in the upper left corner but unresponsive.  As is the keyboard.
I've tried all the suggested fixes on the HP site to no avail.  No combination of Esc, Del, F2, F10, Win+V, Win+B, etc on booting changed the situation.  There are no beeps and the keyboard seems to be not sticking.  Booting with USB KB and mouse is the same because of halting before USB enumeration.  The only signs of life are that a correct label briefly appears for each of the F2 (Diagnostics) and F10 (BIOS) setup if that key is selected at boot.  no menus appear. A few seconds later, I get treated to the language screen with 49 choices and no input method.  And draining all power presents a CMOS checksum error on first boot that is easily cleared by hitting Enter.
Tried to "update" (reinstall current) BIOS with a bootable USB but nothing happens because they haven't been enumerated yet.
What else can I try?
ETA: Please move to Notebooks section instead of Desktops.

[Rogio1]

After a long day of diving into UEFI and EC, it finally started booting.  JustAnswer walked me through the semi-secret EC rebuilding process.