@Jayro , i am having the same issues on a Bootcamp (2014 Mac, High Sierra, Boot authorized) and i'm having the same errors regarding secure boot. I am currently trying on the usb boot creator "UNetbootin" and will try with 'YUMI' multiboot usb creator as well.
When booting on my external SSD Bootcamp Partition, at the boot, i press the "Option" Key to boot on the .EFI partition of Medicat USB, but it can't go past those secure boot errors when prompting an OS or option on Medicat menus.
Do you have any particular instrutions on how to make sure the usb can go past secure boot ? Or add this option on the img?
Thanks for your stellar work!