Thanks to help I got earlier, I am able to successfully use RetroArch to load CD-i games, via the MAME core. But doing so generates a new and unique issue, which I will elaborate.
When I run the game without RetroArch, using the command line, everything works fine. The CD-i game brings up a menu where I have to click a play button, whereupon the game begins. But when I run the game with the RetroArch command, the (emulated) mouse does not respond to my actual mouse. Instead, the mouse pointer is slowly drifting to the lower-right, like it’s receiving bad input. Moving the mouse has no effect. Clicking a button has no effect. When I go into MAME’s tab menu, I cannot change the mouse controls because MAME ignores my mouse input utterly. Obviously something is screwy here. Perhaps some unintended flaw with the MAME core that seemingly doesn’t affect the real MAME? Or maybe something I can tweak in a config?
I should stress once more that the emulator does indeed work. It just seems as though something is preventing the mouse from functioning, and causing it to exhibit false input that drags the pointer to the lower-right. Unfortunately this completely prevents the emulator through RetroArch from being of any use, unless there is a solution to be found.