Hi Jeanne,
You are getting close to the show, how exciting! Great questions. See my answers below and ask if you have more.
1. Do you think it's a good idea for me to take him to the park and tire him out the afternoon before the show, burn off some of that Wheaten energy?
This depends on your history of training with Seamus. Some dogs do much better when tired, others lose focus when tired and the quality of behaviors plummet. Part of your decision-making process in the next three weeks is to find that perfect mix.
2. Do you have any other pre-show/show recommendations?
Proof the behaviors as much as possible and also instill a rock-solid helper, if needed. Proofing behaviors is the process of helping a dog generalize the cue /response connection no matter what the situation. Dogs are notoriously bad at generalizing. In other words, "Sit" in the kitchen might not result in "Sit" in the living room unless the dog has practiced sufficiently. So, you need to come up with a proofing hierarchy based on the variables of performance night. Some are easier to reproduce than others. I think you can find people, noises, lights, etc. by working outside, on busy streets, in dog-friendly stores (Urban Outfitters, for example), you can also play loud music and practice, practice near traffic, etc. The big open auditorium might be difficult, but maybe you can gain access to the performance space at least a few times more than what is scheduled? For a performance where you have a limited amount of tricks that you want performed reliably, you can also strategically increase the rate of reinforcement for his trick(s) and not reinforce his other behaviors as much during the next few weeks. This is a strategic way of helping him "default" to the behaviors that result in a reward. The helper that I mentioned is a way to use a lure or gentle touch to get him to focus on performance night if he doesn't do the behavior on his own as designed. Keep in mind that hollywood dog trainers don't worry as much about fading out treats or getting behaviors to happen without any help. They just need it to look good for the camera. Even if you or someone else has to provide more "help" the night of the show, he will still be a crowd-pleaser.
3. I'm going to try to answer the third question, after listening to you and reading your responses. In addition to turning my back and ignoring Seamus when he crows and jumps, reward him for being quiet before he crows and jumps.
You got it.
We are getting excited for the show.
Good luck! Take pictures, and/or video!
Jeff