Mister_MO and community
I was able to disconnect and remove the screen from the connector in the lid; I did not go all the way to the motherboard and remove the other end of the cable. This did not seem to have done anything on the initial bootup, but after a reboot, it worked; I could see the boot menu, and enter BIOS, but did not see any sign of the Linux GRUB booting menu.
I was able to turn off Secure Boot (but did not purge the Secure Boot keys) and correct the clock in BIOS. The initial screens remained available through a series of reboots, starting in Safe Mode, and shutdowns, while I explored to see if I could locate the GRUB menu. So far so good.
However, after switching to Legacy boot mode in BIOS to see if that would allow the GRUB menu to show up, I got a warning that this might prevent any OS booting. I opted to go ahead anyway. Both OSes can be booted as before, but now the initial boot screens and BIOS are no longer available; I am back to square one!
I completely shut down the machine, removed the battery and purged any residual power by holding down the power button as before, to see it this would allow it to get back into the recognizing there was no longer an internal display. No change on restart; no HDMI output until OS booted.
I find it hard to believe it can still think there is an internal display attached, so I suspect the regression relates to the legacy boot selection. However, I also opted to change action key operation so that Fn + [action key] is required, not just the action key, so the regression may be related to that, although it seems less likely. I don't want to risk blindly trying to navigate my way back to the BIOS section to turn off Legacy Boot again; too many steps and thus room for seriously stuffing up the BIOS.
Maybe the simpler steps of navigating blindly to reset the BIOS to default settings will restore UEFI booting? I think from memory that would involve getting to the BIOS screen (Esc > F10) then moving to the most right hand menu item by one [left arrow] key press (because I am not sure how many [right arrow] steps would be required). Then, two [down arrow] key presses should get me from Exit Saving Changes > Exit without Saving > Reset to Defaults. Now pressing Enter should complete the reset, unless there is a confirmation dialogue related to this action.
If anyone could confirm that my assumptions and these steps are accurate or not, that would be a great help. The BIOS on this HP15 is Insyde F.23-09/19/2014, and SMBIOS version 2.7 if that is helpful. I am also curious about whether anything I have changed in the display settings in either OS (e.g.; whether there are one or two displays, and which is primary) could have any effect on the display selection during boot/BIOS entry, before any OS has loaded. I suspect not, but would be nice to confirm this. I also wonder if the changes in BIOS can limit or change what the OS display options are now. I ask because there seems to be some changes/limitations to display options, particularly in Win 8.1, such as not being able to get letterbox effect as before when resolution set to recommended optimal for this display.
Many thanks for any ideas.
Cheers