For anyone coming across this thread, the [Wii]Mario_MULTi_ROM_Package[PAL_NTSC][WIISOS.com] ISO is actually a GCN ISO and not a Wii ISO.
So this is why it gave me issues.
As of now, and probably indefinitely, you can not load GCN ISOs from USB.
This GCN ISO must be played from DVDR, and then you must use GCN contollers and memory cards to play it.