Well, I made the right choice. Laev bombed her track on Saturday, which was I think part loaded-for-failure (as I said, we weren't going in good shape) and far more lack of preparation. She's tracked another person only three times in her life, and she triple-checked the start of the track, which is very unusual for her, so I knew she was uncertain. Once she got off the track, she kept casting as requested, and I could see a slight hesitation as she crossed the track -- but she was looking for my scent in a field where I'd never been.
I wasn't upset; why worry that my dog didn't do something I hadn't trained her to do? We track over other people and crosstracks all the time, and Laev has learned to look for my scent among others. I need to enlist other tracklayers, and Laev will get it quickly. I'm not worried.
More importantly, I've had time to think about re-training. Laev is talking to me again ;-) in that she played with me on the training field (no gunfire) and, while not nearly where she was, is performing her obedience with more enthusiasm. I also set her up for two long downs, traditionally her worst exercise (because *I* hate training duration), and she never offered to budge, even when a loose dog headed for her and made her tighten anxiously. (The dog was called off before reaching her.) I think part of that success was a change in location on the field; there was no history of gunfire in that long down geography. But it's something I can use.