If you don't trust your coach, that relationship is over. Coaching is not about writing workouts, they all do more or less the same thing, there's no secrets. So after that, it's all about the trust between athlete and coach. If an athlete doesn't trust what their coach tells them to do, which races to run and how to run them, then it's over. In that respect, it's not a surprise that SML ran all-out for 300 if that's what Kersee told her to do. The time to re-evaluate for SML will be at the end of the season - did that crazy tactic in Paris set her up for something bigger later in the season?
Personally, I don't know why she didn't just do that in LA rather than scratching. She'd have been able to take that knowledge and adjust for Paris. I think Kersee got that wrong but time will tell
You wrote that Sydney needs to trust what her coach tells her to do, and you simultaneously said that Kersee made a bad decision. You can't have it both ways. Either Sydney does what Kersee says or she doesn't.
Clearly Kersee's instruction was for her to take it out at WR pace to 300m and see what she had left. This would give him valuable information that they simply couldn't get in training. He now knows what they need to refine.
Adjust that 22.6+ she opened in here to 22.8 or 22.9 and that will make a significant difference in the latter half of the race, possibly even a 1 second difference across the whole race.
Yes, that would mean 48.7, nowhere near 47.6...but she isn't fully fit or peaked yet. If she does run the 400m in Budapest, and the weather Gods are good to us, I think she will run around the same time Naser ran in Doha. No WR but an amazing time.