At this point, I honestly do not have the time to complete this game as it would require all of my attention to even go somewhere with it. Instead, I'll focus on learning the RPG scripting and create a small prototype demo with the RPG mode so that I have a grasp of that. Once I can show to myself that I actually know something about this engine, then I'll focus on making a true game. Whether that's Dark Tunnels, or another project, I don't know. I'm beginning to get the impression that I would need to move on from Sandbox in order to finish Dark Tunnels (perhaps I'd use Unity?) but I definitely want to create an RPG with Sandbox, and at the very least, that is what I'll focus on from now on.
I'll do my best to whip up some screenshots once I've got the idea down for the prototype. Thanks guys!