Sounds like you got it right in your final run. Just to run through your post,
In order for sideload to work in TWRP you would need to do Advanced > ADB Sideload.
Your instructions were “Please flash it through the recovery”
The 2 options I would consider are copy with a PC to SD card or adb push to /sdcard or /external_sd.
In TWRP at “Password” when you know no password exists, Cancel 
“Swipe to Allow Modifications”
I choose “Install” and navigate the file browser to startlte-firmware.zip
I think you covered:
“Swipe to confirm Flash”, you do not want signature verification.
In this case “Automatic reboot” is ok.
There is a set of images in this thread - does your phone deviate from those images / method? B) Transfer the /e/ ROM to the internal storage of the smartphone. (At this point in the tutorial, we are loading a ROM, but same sequences would apply)