Addressing your issues in order:
Regarding the last part about legacy/BIOS aka “compatibility” mode and not EFI/UEFI, you are booting in EFI/UEFI mode (which does work) then attempting to install with/in/from EFI/UEFI mode (which does not work and is an upstream issue with the OS that it is based upon or was made/created from) which then results in the installer crashing at/towards the very end, because it does not know what to do or how to deal with/handle the EFI/UEFI boot configuration stuff (despite it’s live installer system being bootable and you reaching the installer from within that mode).
Everything that you needed was already documented in the thread (that’s where these quotes all came from), or just ask. Anyways if you really need EFI/UEFI mode over legacy/BIOS you can always use/try Ubuntu Server or try the JM36 image again in that mode except at the end once it crashes start up/try out “boot-repair” afterwards and see if you’re able to fix or “complete” the installation afterwards using/through that since really the only thing missing is the boot configuration when the installer crashes in that mode. I personally have not tried it but if/when I do if it works I’ll create a new revision with that already done.