Hello again,
Very well. To answer your question, I've went ahead and disabled each and every plugin I have, one by one, to figure out which one it seems to not want to co-operate with and I think it might be Visustella's Message Core: Message Core VisuStella MZ - Yanfly.moe Wiki . (There could be more incompatibilities with their other plugins that I haven't encountered yet).
Whenever I place your plugin before or after it, I still get the same error message from my first response. I kind of need the Message Core enabled for other aspects of the project I'm working on. You can read more about it on the wiki link I provided and see what it does since it's a very complex plugin. I think it might even have a free version you can look into too from their sample project that should explain what it does to see what's causing the error to occur with your plugin.
I apologize if I come across as a nuisance, I just really wanted to get this plugin working and have unfortunately encountered these obstacles in my way that I thought I could get some help for. Hope to hear from you soon.
It's similar to how HIME_ChoiceDisplayMode used to do it in MV where the choices would be embeded inside the textbox. For whatever reason, since I have the "Relative to message" feature set to true, it looks something like this where the choices seem to be off the text box: