I’ve looked into this issue myself and haven’t found a fix.
Don’t give up on your idea of NPC’s just quite yet though, I’ve heard talk of some “V2 Devices” (THREAD: Community Concerns about V2 Devices) that are going to be implemented very soon, my guess is version 25.00 (Chapter 4 Season 3) which should be released in early June. As implied by their name, I’m guessing that these “V2 Devices” will be a complete rework of the devices currently available in UEFN. I can only hope that they will include new NPC-type devices that might allow creators to use any character they want without issues.