Try booting from the USB by adding bm argument to the command line:
boot usb1/disk@1:,\boot.img bi umsd0:morphos.iso bm

That should give you a boot menu and if it's displayed, you can try to boot with some standard lower resolution modes by selecting them from the Display options. Maybe one of those would get you all the way to the desktop view.

Mac minis aren't really designed for big resolutions and you probably should use some lower spec modes, which probably need to be set up by hand after you have installed the OS. Maybe auto detection just gives you too big values with your current displays. Even FullHD seems to be on the limit in some cases.. for example I haven't had much success with FullHD modes with my Mac mini with 32MB vram, but OTOH my 64MB mini seems to work fine with FullHD.

In my experience TV setups are also quite picky about the correct modes you offer them, it may need lots of testing to find suitable modes. Real monitors seem to be easier in this regard.