I've decided that I want one of my characters go through an ordeal in which some folks he knew before the trial of rebirth have come back to mess with him. I initiated the plot by discussing some odd happenings with one of his house leaders, and that person took it to the next level by going over some ways to help solve the problem, including getting more House members involved. I like where this is going, but I'm afraid it can only go so far because part of the solution is to confront the people from the character's past, which I can't do because they don't really exist. I'm worried that it might just drop off into nothingness, which will cheapen the whole experience and not be fun at all.

Has anyone ever done something like this? What ways can you think of to help circumvent such a problem?

I'm not following 100% and don't actually understand what your problem is. I'm not being snarky-Danith when I say this, but... I think you're thinking too hard about it. Just do what is fun and engaging for you, don't worry about the realism or whether or not it leaves open plotholes in your background.

What he's saying is that the stuff he's doing wouldn't really work out well without it culminating in him (and likely others) confronting the people from his past. Because said people are non-existent, there's a bit of a problem seeing as he's playing with others, who likely won't understand the idea of suspension of disbelief, playing along, etc.

Possibly an idea here could be like emoting/illusioning these people talking (thinking from an ooc standpoint). It's kind of how I did a ritual involving me fighting something I can't possibly have anymore.

I can understand not wanting to post all the details on the forums, but without knowing more it's hard to know exactly what you should do. In a lot of cases, it might be best not to actually emote anything out, just say that you did whatever it is you need to do, assuming it's something your character could do without any other actual players present. Snareman's suggestion could also work, depending on the circumstances.

In nearly every case, you will be worse off messing with stuff before your character was 18.

This too, it's discussed in another very recent topic.

Edit: Here

I will agree that it's usually bad to go messing with your past- it can even be a little odd with families. For instance, I had my parents be dead denizens to avoid the issues there.

If you've already started and need a way to finish, I'd suggest either emote/illusion the people or email roleplay@achaea.com with your problem. You might actually end up getting basic denizens if you're lucky, or more likely room visions of greater authenticity than illusions.

Mostly thanks to the help of Beneficia, the project turned out to be a stunning success. I avoided having to confront the imaginary denizens at all, and still came out on top with a fullfilling RP experience.