[Honor 7X  Questions & Answers] aPlanetaryCitizen:
No worries, I managed to fix it anyway. Some things might be obvious but I'll share what I did nevertheless, in case some other newbie finds himself in a similar situation.

So there was seemingly no way of reinstalling twrp nor the stock recovery. But luck came my way and by almost random clicking I found out that, even if multitool closed as soon as I clicked the ok button in the error message, if I simply ignored the error, the program worked just fine (as I write this I installed another version and it gave no error message). So I played with it a bit and found out that it can flash twrp and it does not need a file from you, it already has a twrp version ready to install. I did that and it worked. It looks like the first twrp file I used didn't work anymore or was someway corrupted by the countless things I tried. So, with twrp working again I was able to backup the sloppy openkiring OS and try the unbrick tool. So I browsed the internet for a guide. Guides always mention as the first step for unbricking using the VENDOR.img file located inside a full stock rom. Problem is, 99% of the times they provide a rom without the needed file. So after some tries I found a good complete stock rom with the VENDOR.img here: drive.google.com/file/d/1tjHefytEWbpuV1f4YjX8AaSMzT6jr8TQ/view but I can't remember which forum/site I found it in.

I followed the guide which involves:

-extracting the UPDATE.APP file contained in the provided zip with huawei extractor tool into an X folder on your pc

-copying the VENDOR.img file contained in the X folder, on the root folder of the sd card

-backing up OS (if there's any) via twrp (I had something like 50 partitions, I guess that was messing things up a bit, but for safety I backed them all up)

-doing an advanced wipe from twrp selecting ALL checkboxes BESIDES the SDcard of course

-flashing the vendor.img file via twrp selecting the "Vendor" partition when prompted

-downloading the update_full_[your model]_hw_[your region].zip file, in my case update_full_BND-L21C_hw_eu.zip

-extracting the UPDATE.APP file contained in update_full_BND-L21C_hw_eu.zip with Huawei extractor tool in some other folder Y

-looking for a file named cust.img/CUST.img/version.img/other_name.img (I don't know which other names it can have, I was lucky and had a VERSION.img file in it)

-rename the cust.img/CUST.img/version.img/other_name.img to CUST.img and copy it from the Y folder to the X folder

-boot your phone into fastboot mode

-connect phone to multitool via usb and select the "unbrick" tab

-there, Multitool asks for a folder, give it the X folder. There are some checkmarks on the Multitool "unbrick" page. 4/5 of them should become checked after the folder input. In my case the first 4

-then, simply press the unbrick button

-wait and pray (it can look like it freezes while it's copying the system.img file but wait, it'll be fine)

So I did that and it worked flawlessly, the phone booted slowly but surely into EMUI 5.1 with Android 7.0 I think and everything was fine.

At that point I had a working OS and a working twrp but wanted to go back to the OS I was used to, Android 8.something so, I tried connecting the thing to HiSuite and it automatically asked if I wanted to update, not to 8.something but to 9.1. I thought "SURE I DO!" and updated flawlessly (I thought 9.1 hadn't even been released here in Italy).

When it booted I found out the new OS was working like an antivirusless xp pc: it didn't recognise the SIM, it would crash every 20 seconds, app downloads got stuck at 99%, if it felt like it, it rebooted, pc connection was not stable. Besides, the update being "official" and installed from "trusted" sources (HiSuite), it completely wiped twrp.

And that was the "OH ****, here we go again" moment: no twrp and worse-than-ever-working OS.

So because of the new EMUI version, I guess, Multitool kept recognizing the phone but stopped offering its own recovery, eRecovery, twrp files available for flashing. I was completely frustrated with all of this and decided to give up, but I made the last effort: I tried the relock function in multitool. So I provided the key, pressed relock, waited, prayed, and finally cheered. It worked like a charm. Phone runs now on EMUI 9.1 with locked bootloader without problems. Finally

So I guess in the end I payed 4 euros for a nice update, a painful and frustrating trip into android modding and a lesson : never do android modding!

no, just kidding, don't do it just if you are a newbie AND you don't want to risk to throw the phone in the grabage.

In all other cases consider it carefully ;)

Oh, and also, God bless the MT Team and Russia altogether!! thanks