Yeah, I had the same problems... Jagex has fixed the NPCs so that they appear over any imps now. You should use your minimap to locate the NPCs to talk to. Use the Ice amulet from last year to summon your imp or walk to an open space to talk to it. When I did the event, I had 3 friends in the same world online doing it at the same time so we could help each other, plus I was in a cc where further help could be found. Still bloody frustrating though, the glitch with the reward is way more fun than the entire event lol.