Hard shutdown broke my system

[tzwcfq]

@fsflover

Then I could mount the partition, but file /mnt/etc/lvm/backup/qubes_dom0 does not contain any mentions of 8558 and the first transaction_id is actually 8560 (and later, many other numbers, too). So I don’t understand what I should change in this file.

You can try to follow this guide:
https://blog.monotok.org/lvm-transaction-id-mismatch-and-metadata-resize-error/
After you create volume config backup with:
vgcfgbackup vg-fed -f /home/<user>/backup
Open the file /home//backup and change the transaction_id in qubes_dom0 { logical_volumes { vm-pool { segment1 { transaction_id = XXX } } } } block:

qubes_dom0 {
...
        physical_volumes {

                pv0 {
...
                }
        }

	logical_volumes {

                vm-pool {
...
                        segment_count = 1

                        segment1 {
...

                                type = "thin-pool"
                                metadata = "vm-pool_tmeta"
                                pool = "vm-pool_tdata"
                                transaction_id = XXX
...
                        }
                }

Change vg-fed to qubes_dom0 and thin-fed to vm-pool in the giudes commands.

@fsflover

By the way, I had to perform a couple of impossible tasks for that: choosing an ultimately trusted USB stick and an ultimately trusted live distro… Is reinstalling always a more secure action in such cases (so lost data might even worth it)?

It’s a good practice to keep a trusted USB with Live system for a system rescue.
Since you’ll need to create a trusted USB with Qubes installer ISO to reinstall the system anyway then you can create and use it for system rescue when you boot from it and enter Troubleshooting → Rescue a Qubes OS system in grub.