So, out of curiosity, I've been attempting to see if I can patch the shareware HERETIC1.wad file to play all three original episodes, and I'm running into this error: 
So, my question is, is there a good lump editor that I'm missing? How would I go about fixing this?
The HERETIC1.wad does not contain the assets for the other episodes. That's not going to work.
derSammler wrote on 2020-04-15, 06:13:The HERETIC1.wad does not contain the assets for the other episodes. That's not going to work.
By default, no, it does not. However, you can very easily add in the missing files needed, if you happen to have a copy of the game on Steam.
And yes, it does work. I suspect it's a lack of resources/assets that prevents you from playing past the first episode in the shareware release. Going to tinker around with it more tomorrow.
After a bit of poking around some more, I've determined that only one file in the wad needs to be removed to 'downgrade' from 'Shadow of the Serpent Riders' to base Heretic. This thread can be closed now.