I didn't read every response, but I can tell you that our entire first YEAR of showing felt a lot like this. Everything felt rushed, nobody knew quite what was expected, and as the person in charge, I'd say one thing once, and then change it on the poor kids (bad on me, I know). The second year was MUCH better, and now, in our third year, shows are easy. I mean downright EASY.
One thing I noticed in your original post is that you said your trainer said your fitness had improved, and yet by the end of the day, you were worn out. Try to keep in mind that a lesson is about as long as 2-3 classes. So the fact that you're in good enough shape for a lesson, does NOT mean you're in good enough shape to ride 6 classes in a day without getting tired. If its possible, try riding twice a day for maybe 3 days a week, so you're putting in that extra time before hand, and getting used to it. Another idea is to go for a long trail ride where you work on your position, your half passes, yields, etc etc. during the ride. An average (for us) 3 hour trail ride will approximate the amount of time you'd spend in the saddle on a show day, and if you ride correctly for the entire ride, you'll have a pretty good idea of how you're going to hold up when every muscle movement counts.
Another thing that might help you is to keep in mind that your trainer is human, and possibly even more stressed than you are. The 'smile and nod' strategy works wonders. When she says something that seems snippy, no matter how much you want to snap back (or cry, depending on your personality), just smile, nod, and say ok. The show is never the place to discuss things, so keep a mental list (as you did) and possibly a paper list of things that didn't work for you at the show, and things that bothered you. After you've all had a week or so of normalcy, approach the trainer and ask her how you can do better in these situations. Explain that she seemed upset with you, and you don't want to cause her stress, and ask her advice. (This doesn't mean I think you were wrong, because from what you've said, I certainly don't, but it does mean that if you think your trainer is a good trainer who has helped you, you owe them respect in your dealings, even when they're wrong.) I know that as the person in charge of our club for shows, if somebody explains what happened and asks how they can do better, it often lets me see that I was wrong, without feeling attacked.
Once you know exactly how long it takes you to tack up and get ready to warm up, talk to your trainer BEFORE the show, and say something along these lines: "It takes me x minutes to tack up and be ready to warm up. If my class starts at 4 p.m., what time would you want me in the warm up, so I can know what time to begin to groom and tack?" Make sure you take a notebook with you when you ask the question, and when she answers, write it down, thank her, and assure her that you'll be ready on time. If the other riders who ride for her are your friends, talk to them about doing the same thing, so the trainer isn't stressed about everybody being ready in time, because everybody knows just when she'll want them, knows how long it will take them to get ready, and has written it down so they won't forget.
The first few shows are always stressful, but they do get easier. Having fun should be less about whether everything goes just the way you expect, (or the way your trainer expected) and more about knowing you and your horse are doing your absolute best! If after a few (several) shows, things aren't getting easier and you still feel stressed, try either finding a different trainer, or going without one, if you can.
One last thing, just because your horse has a naturally willing nature, and its possible that anybody could have taught him to jump, doesn't mean anything. Nobody but you taught the horse to jump. You should be very proud of yourself AND him. You did a great thing, regardless of what anybody else could have done. Don't go doubting yourself now!!