You will need to rotate the webcam settings in mainsail by 180 deg to correct the feed.
I used the K1 helper script to set up mainsail and full-featured moonraker on the printer along with support for third-party USB webcams. You can follow the instructions after updating the firmware to the latest version v1.1.0.14 from the printer and enabling root (do not mind the firmware warnings for K1 and E3V3 from that wiki; it works with E3V3KE firmware as well)
The author remixed this model.
Remixed from this model reducing the thickness on some faces and modified the mount for my webcam with a ¼" threaded mount.