I flashed the engineering bootloader, rebooted without OEM unlocking and got a red triangle. Auto reboot into the system afterwards and no red triangles going forward but even if I use qfil to reflash the engineering bootloader I can't seem to get anywhere near fastboot mode, either going into lgupdown, recovery or the system
I thought my abl would be corrupted or otherwise inaccessible potentially
Well, here's something important to know and realize it's a given; If the phone boots, then the abl isn't corrupted. ABL stands for Android Boot Loader. Phone don't boot if that don't work.
What probably happened, is u flashed the engineering abl, and didn't do the remaining steps, which the last one would have been to flash your original abl back.
The point of the eng abl is one thing: to unlock the bootloader. It doesn't boot the phone unless the rom you have is Oreo. Yer probably not on Oreo. So the phone probably tried the other slot, and that's prob why u got the same update as before.
It also sounds like u don't know what yer doing. I'd recommend re-reading the guide, slowly. numerous times.
If a step doesn't make sense, ask questions about that step. One step at a time.