Participation overlap between concurrent or adjacent scenes is one of those things that always irked me as an ST. That being said, I think "arrival" scenes are a whole other animal.
It is very common for arrival scenes to happen (in real-life time) not only immediately before, but concurrent with, and sometimes even well after the start of some other Scene of ImportanceTM, such as a challenge or gathering for the departed. It happens all...the...time.
The problem is that although the arrival scene is a necessary prerequisite to the Scene of ImportanceTM, it is no less an opportunity for fun and interesting roleplay than any other scene. This can, and does, frequently cause arrival scenes to run long. I've seen arrival scenes that took longer to play out than the reason for coming.
As long as there are no "dire consequences" involved in an arrival scene, I've never had a problem with characters proceeding directly to the Scene of ImportanceTM as soon as they are clear to enter and do so. This only requires players to be cognizant of the progress going on in the Scene of ImportanceTM, if they choose to continue the arrival scene also.
To use the current case as an example, now that 7M has left the arrival scene for the mound, others in the scene who plan on being present for the challenge ought to see that it's time to do the same. I see it as a common sense, self-induced wind-down. It's easy enough to say "Hey, we should really head to the mound, lets continue this conversation after the challenge?" then start a new scene for the continuation once the challenge is concluded.