What Quentin did not spill the beans to Petofi about was that David Collins believed that Quentin was dead. I imagine that Quentin felt it was immaterial, but Petofi, with his interest in going to the future, would be assuming that David saw Quentin alive, and wondering how Quentin made it to 1969 still looking young.
When Barnabas told Quentin that he didn't know how to get back to the future, I felt that we were watching The Wizard of Oz, when he left Oz without Dorothy. "I can't go back! I don't know how it works!"
Is that cabinet TV a one-trick pony? All it ever purports to show is people's death. As for why Petofi showed Beth the vampire-induced death - maybe he did it just to throw dust in people's eyes, and make Beth, Barnabas, and Quentin believe that it was by Beth's own choice that she was freed from the vampire's thrall. I hope word gets to Trask about it; I'm sure he'd like to know that victimization by vampire is a choice. By the way, as EmeraldRose noted, Beth has been told that she would die by suicide and that she would die by vampire bite - but we've also seen Quentin seeing a pentagram over her face. But that's life at Collinwood: you've got your choice of nasty deaths.