Sorry about the pessimism that comes from some people on this forum.

Anyways to your question. Will Jack disappear if you "get into the higher levels of lucidity"? It is possible that he could, but it is recommended that oneironauts be just that: explorers of the dream world, not the ultimate controllers. If you keep yourself in a passive position in your dreams, you will be amazed at all of the creativity that pops up from your subconscious. Granted, it is expected for oneironauts to exercise their control over dreams, but not too much... But you may do as you please for the above is solely my opinion.