This is a great topic, excellent read
I'm kind of on the Gavin front here, but I think that it's less subtle than he makes out.
I've been reading (for example) Joey and Hixee's posts since they were "school teens". At no point have I ever looked at their posts and thought "there's somebody who will spend their lives flipping burgers". For a teenagers, they (and a lot of others) come across as thoughtful, intelligent and with a "background interest" in the hobby. They know the stuff that goes on and understand how it works behind the scenes. For a Joey or Hixee to post aged 15 "I'm working towards becoming a theme park designer/engineer", then you'd believe them and not shoot them down - simply because they not only "get it", but because they have intelligent thought and understanding.
When you see A-Kid's design work too, the stuff he does for "fun", you realise he has a creative talent and as Beans says, he's working through the system and has always understood how it works.
I guess the biggest issue is when people who have only been to Thorpe/Cedar Point come on and issue a statement like "i'm going to get a job in the industry making coasters because their really great fun to ride and i've made a new element on no limits that is well wicked". Then you fall rapidly into Gavin's point that they probably should be shot down, or at least ignored because it's going to readily come to light that it's a passing fad and the golden arches are calling them to a career in a hair net.
I mention Joey particularly, because (due to his dyslexia) his posts are often hit and miss in terms of English, it's not just about poor presentation of a post; if there's a really good idea or thought behind it, it shines through. So it's not just a superiority thing based on spelling/grammar (just thought I'd make it clear).
Other than that, the advice Joey gives is excellent and spot on, and I think it's a real shame that things haven't panned out for you so far Mark - often there's a lot of situations where time, money and location are just completely against you. It's gambling your life against a possible foot in the door for the future; something that you can only happily do if you have little to lose.
Have you not thought of approaching the design house at AI Mark to get into the industry?