Prohibit symble after Mojave minor update, and cannot type password to unlock disk during reinstall (unlock iCloud Recovery User)

timeline

1. at the end of 2014: purchased this macbook pro 15" , created user A and started using.
2. around 2015~2016, enabled FileVault after a major update (Sierra?).
3. july 2019, updated to mojave and then created user B (also admin). till now, everything was ok.
4. septermber 2019, the system pushed a minor update (a version of 10.14.6?), and I clicked "update now" under user B. it popped up a prompt saying that another user (user A) has logged in, but I clicked ignore and proceeded with the upgrade.
5. after a reboot, the screen shows a prohibit symbol and stopped at booting.

solutions tried and failed

Tried the following after all sets of PRAM reset, SMC reset, etc..

1) tried to start a reinstall follow the official HT204904(press command-R or option-command-R at boot), but when it comes to unlock the disk cannot input password, same problem discribed as in this thread. the keyboard, mouse and touchpad worked fine, but i just cannot click or focus on or input anything in the input password box. video can be seen here https://www.youtube.com/watch?v=9BDf566nUA0

2) press command-v at boot to check detailed log. it shows error info:

BiuldUser(): error building a user of type 0x20010008
ERROR: no panel or special users!
No CSFDE users processed
...
ERROR!!! Load prelinked kernel with status 0x800000000000000e
Error loading kernel cache (0xe)

3) tried diskutil referred by online solutions (1, 2, 3). the problem here is there is not any things like personal recovery user. there are two Cryptographic users found:

+-- ID1
|   Type: iCloud Recovery User
|   Note: Unlock with iCloud account data + iCloud Recovery External Key data
|
+-- ID2
    Type: iCloud Recovery External Key
    Note: Stores partial credentials for the iCloud Recovery User



I tried all passwords I know (diskutil apfs decryptVolume -user), but none went through.

It seems that the updating program failed to handle encryption data. It is kind of ridiculous that a small update can crash the whole system (that cannot be recovered).

Too bad I do not have a recent backup for this computer, so I don't want to erase the disk. How can I restore/reinstall without erasing the disk? How can I unlock with iCloud account data + iCloud Recovery External Key data?