1. It is certainly possible, but I would expect that it would have to be an extremely special case. There is a note about it at the very bottom of the Illusionism article.
2. The Vexation article specifically states that, at the first level of the Gnosis, the user has to directly injure someone via touch. If you had the power to cause such vivid illusions that pain was actually felt, I would think this still wouldn't count as a direct injury. The second level is a bit more flexible, but the wording still seems to specify that it amplifies the pain of existing injuries. However, when it comes to killing, I do not believe it specifies that the kill has to be a specific sort of death. To be honest, it would take quite a lot of time to get both Gnosis up to the level where you could consider this sort of thing. I'd question the two deities even marking the same person.
3. I think it would be very hard for a character to instinctively stumble upon a magic. I think the process of advancing and using Hypnotism is very instinctual, but the idea of getting lucky and just 'figuring it out' seems far-fetched to me. However, I am no expert and some of the more magically-inclined players would be better off answering this.
No need to apologize. Asking questions in the correct forum is certainly a much better option than just taking a guess.
