[General Questions and Answers] SubwayChamp:
Do you refer to this? https://mega.nz/#!NYF02CJa!DM93FpT1T5XurOd8XJ8pWpXvmSnZo6a1w4lQQDpFEBY I think as you said that this firmware is for engineering testing purposes only with a reduced number of apps and tools to check some hardware status and of course it could work to unbrick, repair and also to bypass FRP status (probably not developed exactly for this but..)

Samsung enabled from oreo a new security measure; RMM that make OEM unlock inaccesible for the user initially during 7 days from the first setup (prenormal state) this is a link to enter to the Samsung official account where RMM state can be removed and probably doing this way this can be fixed permanently without need of mark it on every reboot (probably this was the error, they didn´t care about this before to attempting to root it and device entered to a permanent RMM prenormal)

https://rmm.samsung.com/

Also there is a patch that can be flashed from TWRP to fix this (version 2 for RMM) https://drive.google.com/file/d/1UvqcYUbVTFomAmKCVkMyfTWeuz1sEz8n/view

And here the new version (v3 for KG-KnoxGuard for pie) https://xdaforums.com/showpost.php?p=79745844&postcount=627

I saw firstly this kind of exploit as reversed engineering for example on devices like Galaxy Note 4 from AT&T carrier locked that worked with a modded boot called ENG boot something like this https://www.fullstockfirmwaredownload.com/downloads/eng-boot-sm-s320vl/

In my particular case I ever did go for the sure way buying a device that can avoid this but it should be needed more research to do something similar that could work for your device I mean I was familiar with using it in the past but the things from Oreo and most likely on Pie changed.

I actually have a similar device, J7 Pro (also called 2017) and like I read a bit before to do some step I didn´t update it when at first setup it prompted to do so, by fortune I stayed on binary 6 so the things are a bit complicated on v7, I mean that Samsung implements on almost every update new rules to the game. But despite that on v7 also can be rooted if the steps are observed strictly. I also think that Samsung put the best effort on S and Note series and not the same on the rest of devices excepting those bootloaders carrier´s locked.

In my understanding this will be the route to follow towards the goal of rooting new Galaxy devices and I´m absolutely sure that should work too onto your device:

The first is bypass RMM/KG security implementation otherwise bootloader will act as locked ever.

The second step is not only root it but avoid that other security implementation like Defex be activated, here´s a fix that works on many models:

http://www.99mediasector.com/how-to-disable-defex-security-to-root-samsung-galaxy-phones-oreo/

And third; bypass through DM-Verity, as you know this implementation is from Google to check the integrity of whole image, also exist some modules that can do that, the v6.0 looks like the best matchable for various models, DM-verity on XDA:

https://xdaforums.com/android/software/universal-dm-verity-forceencrypt-t3817389

no-verity zips ready to flash:

https://androidfilebox.com/download/1977/

If these steps are carefully followed I think that J7 models can be succesfully and permanently rooted, of course even that bootloader is not locked by the carrier otherwise complete firmware has to be changed.

Regarding the stock recovery flashable for the member that you mentioned probably you can find one from a similar device and replace what it needs (script and images) can be done. Sorry If I couldn´t explain myself in a satisfactory mode, let me know if I can help on something more.