[Huawei Mediapad M3 Questions & Answers] xdaens:
I have some time tonight and i will try again following clues you just posted, i'll post an update in a few minutes (hours? ^^). thank you for your time, i clicked the thanks button ^^

first edit:

you are right, the OTA (or « manual OTA » with dload ») won't work for the reason you explained (basically because twrp is not the expected recovery that can handle OTA images), that's what i guessed because i had the same problem recently while trying to OTA-update my twrp-rooted nexus 7. As i want (and it's the only one available yet) the official OTA nougat, the original recovery must be set back in place. (for my nexus 7 i ended using an alternative ROM instead).

So, following your first solution, i went back to this page https://xdaforums.com/mediapad-m3/development/twrp-greatslon-mod-t3489232 and at the bottom, https://androidfilehost.com/?w=files&flid=125388 , this is where i applied the twrd at first place, for the story, when i tried the twrp.zip as mentionned in the post i softbricked my tablet, but i managed to make it work with twrp_BTV-DL09_greatslon.img.

Anyway, as i already have twrp now i guess there is no use trying one of those twrp other img trying to remove it, and the only image that looks like a « rollback » would be:

emui4.1_6.0_M3_B005_greatslon.zip

i am not sure it is for my tablet version (BTV-DL09) and even appart that, it's not the nougat version (because you mentioned using greatslon nougat version then the official rollback image i saw before on huawei italy). my guess is that it would at best work on my tablet but without removing twrp, because only the official rollback package will remove it?

in the mean time i am trying to look for the nougat greatslon firmware now. to be continued.

edit2: stopped the emui4.1_6.0_M3_B005_greatslon.zip download,

just found what looks like the russian page you mentionned: http://4pda.ru/forum/lofiversion/index.php?t780572.html i am downloading BTV-DL09C233B338 (FullOTA) + data

edit3: the download is very slow (2 hours expected), so i am trying to understand the « options » i have here, on the russian forum.

I noticed the Android 7 section, all the downloads are marked as « FullOTA ». the « data » part seems to be for the chinese version, i think it won't work for me.

then there is a « EMUI5.0+Android7.0 rollback to EMUI4.1+Android 6.0 » package. i guess this is the package i will have to run AFTER i successfully installed one of the android 7 image, but at this point i hope the data part won't stop me.

If i guess well, only this rollback package is designed to remove twrp, but i am not still sure.

In the Android 6 section, i can see a « BTV-DL09C100B009 (Full) » image which happens to be my actual version, and some other versions with various tags, i am wondering if one of those would remove the twrp recovery.

Waiting for the download to end, i will try to understand the difference between: « FullOTA + data », « FullOTA», « Full », « fastboot».

edit 4: because it's a small rar file, i just downloaded the rollback package, there is a instruction.pdf inside, and it specifies it will work only on an official android 7 rom, unrooted and unlocked. not sure if it will work.

despite the russian forum being posted by greatslon, i noticed the « FullOTA + data » download is from the official site too (hicloud), this makes no sense because that's where i downloaded the nougat images at first place and it failed with « signature check » error.

edit 5: just found an english version of greatslon post, reading now to get clues https://w3bsit3-dns.com/forum/index.php?showtopic=780572&st=720