Ubuntu 18.04.1 installer crashes after "Could not install grub-efi-amd64-signed to /target/" (sic)

[TEN]

i5-4200M (2.5GHz*4) 64-bit Lenovo ideapad, 500GB HDD

https://bugs.launchpad.net/ubuntu/+source/grub-installer/+bug/1497033 requests "If your install crashes, allow the installer to file your own bug report with your own log files" i.e. not a duplicate

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
Date: Mon Oct 29 22:53:21 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[TEN (launchpad-20-ten)]

Workaround from https://ubuntuforums.org/showthread.php?t=2346196&page=2&p=13723151#post13723151 no longer applicable to 18.04.1: fails though online updates (even 3rd party) on during install

[TEN (launchpad-20-ten)]

Conversely https://forums.linuxmint.com/viewtopic.php?t=241379#p1289326 recommends installing offline (as well as making /dev/sda1 FAT32, possibly 0C LBA rather than 0B these days)...

[TEN (launchpad-20-ten)]

Cf. https://bugs.launchpad.net/ubuntu/+source/grub-installer/+bug/1767259/comments/10 re. "required ~200MB FAT32-formatted, bootable partition with an esp flag"

[TEN (launchpad-20-ten)]

For the record, the installer crashes whether online or not,
and even after manually setting the /dev/sda1 partition type to 0C,
but the system in Legacy mode (UEFI-only fails to find media) does restart into a GRUB rescue prompt,
so /dev/sda itself is bootable: just the next-stage config obviously wrong.

[TEN (launchpad-20-ten)]

So this ideapad Z510 came with FreeDOS, ran nicely since and under Ubuntu 12.04 (up to .5) LTS and never had Windows 7-10 on it.

Rebooting to USB and mounting the root partition:

mount /dev/sda6 /media/ubuntu/GUID
mount /dev/sda5 /media/ubuntu/GUID/boot
mount /dev/sda7 /media/ubuntu/GUID/usr
mount --bind /dev dev
mount --bind /dev/pts dev/pts
mount --bind /proc proc
mount --bind /sys sys
chroot /media/ubuntu/GUID
grub-install /dev/sda
Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.

At least /boot/grub at this point got populated with the modules that were missing before.

No partition has the ESP flag and/or file system type EF.
The 18.04.1 installer seems to neither expect this (which it should check upfront rather than near the end of a lengthy install) nor try and create one, or a way for non-EFI (legacy) boot.

Anything into grub-install /dev/sda --efi-directory=/boot/efi(/EFI(/ubuntu)) yields
grub-install: error: /boot/efi(/EFI(/ubuntu)) doesn't look like an EFI partition.
...but these directories wouldn't meet the specifications https://wiki.archlinux.org/index.php/EFI_system_partition#MBR_partitioned_disks of a dedicated(?) primary(?) 100-256MB FAT32 anyway.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.