No, simply install the original script without changing anything.
Then restart the iMac.
If everything worked out, you should have an output on your external monitor via the egpu, and the iMac should have switched to the "weird" resolution.
NOW sent it to sleep mode, wait a few seconds and wake it up. The iMac should switch back to "normal" mode.
The other solution you mentioned can be used in case you only want to use the egpu in apps to render stuff, but not if you want to display something on the external monitor, as far as I understood.
goalque explained both options in the thread I linked, but you have to scroll down a bit to find the "sent-it-to-sleep" method.
Concerning your Octane issue, I have no clue.