Don’t be too concerned about the pouncing. She is trying different behaviors to figure out what you want. It is very common for dogs to start physically interacting with the objects at first. With multiple identical objects she will begin to try and figure out why the “hot” one is special. If she pounces on the others, just ignore. As she problem solves she will be narrowing down specifically why one box gets rewarded and the others don’t. This will help her start to use her nose to find the correct box instead of relying on the visual cue of the boxes. At first, you may find that you need to capture the correct hit instead of waiting for any particular alert behavior. Most importantly, only ignore the incorrect box behaviors. We want her to interact with the boxes…she will learn HOW to interact with them by seeing which one gets rewarded. This will require some experimenting on her part.
Begin with the teaball and let her be successful. Then allow her to watch you put the teaball into a box (just one). Begin holding the box so pouncing isn’t an option. Let her be successful at this point with a high rate of reinforcement. Gradually bring the box down to the ground. Then set the box down and reward her for targeting the box (before she can pounce). If that goes well, add a second box but keep your motion to a minimum. You won’t be walking past the boxes at this stage. This is way to play a transitional game to help her understand the boxes.
With regard to her stress behavior and checking out. Did you have her on her collar and leash? When she stayed at your side, she may have been doing the thing she knows has been successful in the past (good leash behavior). When THAT didn’t work, she checked out. Do you have a harness that she is allowed to pull on? If not, this is something to consider (something different than any other equipment associated with certain behaviors). In the meantime, you could try playing the game off leash in a small controllable space.
Let us know how it goes and post a video if you can.