I think it can really vary, one panel is not comparable to another. It really all depends on the circumstances of the applicants in front of you. When push comes to shove there will be people who will decline, others who will work for a while in the role and then leave and some who will stay in the role. So it depends on the individual. The only advice I can really give you is to learn from the process, request feedback and never put all your eggs in one basket. Best of luck!
Nobody can tell you. The more specialist the job the less people they'll hire so the greater likelihood they won't go high up the panel. However - you never know, some people might prefer a role in a different county, they might use getting on the panel to renegotiate the terms of their present job or maybe so many engineers retired they need 20 right away. Really it's impossible to know the lay of the land.