Hmm, I somehow cannot make this work. But maybe we are talking about different situations. You seem to have run update-grub without the apt resinstall memtest86+ and ended up with a (second stage) grub menu containing only the “UEFI firmware settings”, like @paddylandau here.
I have since reinstalled the Desktop (“Erase disk and install”), to get to a clean slate state; wiped the TPM via the UEFI menu, offline install, no additional settings. On first boot, I immediatly entered the (first stage) grub menu (Esc) and booted the recovery desktop menu entry. I had also removed all console type devices from the VM to approximate @paddylandau’s setup in the other thread, so the boot splash gave way to the console login prompt. And, sure enough, those messages about the ttyS0 service kept spamming TTY1. BTW, one can switch to TTY2 (CtrlAltF2; “send keys” from virt-viewer’s menu) to evade that, which I did.
Now, though, I cannot login with my password.
As for the issue with the manual boot command not working, this is the (second stage) grub menu after I booted to the normal desktop (to confirm my password was correct), ran sudo apt reinstall memtest86+ and added a “Run Ubuntu Core” menu entry manually:
This is said “Run Ubuntu Core” entry; just a copy from GH, in the hopes that this would preserve the environment variables and act accordingly:
Running that gets me this:
Having been dropped back to the grub menu, I then entered CLI mode and input the command manually, as you suggested, @avb66:
Note
error: no server is specified. on my first attempt. I then dusted off what little understanding I have of grub-shell and poked around to find the /boot partition. Still no luck. 
Also, this seems relevant (from my OP):
I don’t know how the LUKS module went missing and why it’s even supposed to be loaded. From what I learnt in the Streamlining secure boot for 26.10 thread, it shouldn’t even matter, because /boot is ext4 and the bootstrap of LUKS happens by initrd magic.