I'm not sure if its Exynos or Snapdragon.Hi there, I can help you out! I have some experience with Samsung.
So first off, I take it this is an Exynos model? Snapdragons require the use of a paid service to get an unlock.
It seems you've flashed stock firmware before but it didn't work out due to Knox. Did you flash all the files including the CSC (wipe all data) file? Also, you could try flashing TWRP and that would trip Knox, at which point Knox is disabled due to detecting system modifications or it will try to rollback your modifications.
Yes I flashed the official firmware with CSC file, I have not tried flashing with TWRP yet (unsure what that is to be honest).
In regards to your developer options question, yes I can enable dev options on the phone, but the issue isnt removing the retail mode application, I actually k ow the code to remove that. The main issue is when the device is flashed and you go through the setup stage, as soon as you connect to Wifi (which you cannot skip) Knox immediately kicks in and starts applying settings, and the phone sets up in retail mode without playstore and restrictions on changing settings or privacy sharing.
The crossroads I'm at is somehow bypassing Knox during set up, so that the phone sets up like a regular stock phone. All the work around videos have ways of enabling ADB then running CMD through the PC and removing Knox before it can do anything but I can't do that because in the videos they can connect to wifi and go back a step before Knox kicks in, on my device Knox kicks in immediately after connecting to wifi and you cant stop it.