1. Yes, write 100 times: there is no /etc/fstab used in LE.
With UUIDs this should work. Only with a second copy attached the same time you don't know which is mounted.
2. A long we don't know why the LE image is not accepted this cannot be corrected. Use what is working.
I've created two additional test images, please test on a spare stick if you see the syslinux boot messages.
At last ! Yep, the gpt img booted in run mode. 
Many thanks to all those who helped, including Da Flex (anon donations work !), chewitt, Jocke.Sve and of course, you.
Some details:
This all is Legacy Boot.
(Btw, Chrome browser blocked both img downloads, so had to revert to MS Edge.)
1. I first Rufus installed the legacy img which showed only one drive, USB (D:), invisible contents. Copied AE 10.0.9 Kernel + System (already had a copy in Downloads folder), changed syslinux.cfg to DEFAULT run, and rebooted.
Unlike your first legacy boot img (which resulted in straight to Windows boot, no messages), I got errors this time after typing in run. See pic attached.
2. Then I installed the gpt img via Rufus on the same USB drive (no manual reformat first.) It showed two drives, LIBREELEC (D:), the boot drive (from Disk Management display), and a USB (E:) So I installed AE 10.0.9 Kernel + System to D: Then changed syslinux.cfg to DEFAULT run, and rebooted.
At the boot screen, I typed in run again, and it showed the AlexElec splash and displayed then a resizing partitions message. I waited until it said something like, "Finished, boot in 5s."
I panicked, thinking that the installer had actually run, wiping out my PC HDD Windows partition ! So I detached the USB, and booted again, praying.
Booted fine in Windows !
I then rebooted from the USB drive. It said repairing damaged partitions (abort in 120 s.) Then it booted into the Dell PC splash, and I got back the USB boot prompt. Typed in run, and it booted into Kodi screen...
Changed Kodi language to English, and rebooted. English persisted, so run mode works fine.
All's well that ends well.
3. Questions:
A. Given my Windows near death experience, is there any warning messages/ abort key when DEFAULT installer runs, re: Windows partition wipeout ?
B. So is this gpt boot going to become the default boot image instead of presumably the legacy boot image now in use ?
1. The kernel file can not be opened. But the legacy image was only created to in case the usual GPT disk is not possible.
2. The size of the used stick is unknown so we start with a small 32MiB file system that is expanded at first boot.
The filesystem repair may have been started because you pulled the stick before any data was flushed.
A. Don't be afraid of the installer, you have to select the installation, choose the disk and then being asked twice if you really like to erase the HD.
B. We do use a GPT image. The changes will be used if accepted by the team.
1. The kernel file can not be opened. But the legacy image was only created to in case the usual GPT disk is not possible.
2. The size of the used stick is unknown so we start with a small 32MiB file system that is expanded at first boot.
The filesystem repair may have been started because you pulled the stick before any data was flushed.
A. Don't be afraid of the installer, you have to select the installation, choose the disk and then being asked twice if you really like to erase the HD.
B. We do use a GPT image. The changes will be used if accepted by the team.
Great !
Let's hope the current LE image is upgraded with your updates after some changes 😉
shippy please test LibreELEC-Generic.x86_64-10.0-devel-20210913111945-3105a40.img.gz on a spare stick.
It is a complete image, don't change anything on it.
The images are (intentionally) deleted in the meantime.
To get a current image booting with UEFI firmware in legacy mode change the (protective) MBR flags:
parted -s <disk or image> disk_set pmbr_boot onYou can boot the install stick on any UEFI device in UEFI mode or old legacy boot only device into installer and press Alt-F3 to get the debug shell. Use blkid to detect the stick and execute the parted line.
At least with diskpart in Windows it seem not to be possible.