Do flares stay together or drift apart ?
Well, you can see the flares drift apart over time if you compare the photos taken during the event. The flares move significantly with respect to one another, demonstrating that they are not all fixed to a single craft.
Do flares all have the same brightness ?
Flares of the same type do. But in this event there are two different types of flare, and the ones that mostly emit visible light are significantly brighter than the ones which mostly emit infra-red light.
Apologies for posting the same image again, but this demonstrates the difference nicely.
Why some flares might light up a minute or two later ?
Because they were launched at different times? Probably from helicopters or aircraft too far away to be audible. These flares were likely about 10km from the witnesses, if not more.
Over 50 Marines told Corball and Knapp they knew what flares looked like, and this was a UFO.
The Marines were presumably not familiar with this kind of flare. They would be familiar with ground-launched flares or perhaps decoy flares, as opposed to long-duration aircraft-dropped flares.
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Finally we know for certain that there was an exercise occurring on this date, and in this direction as seen from the camp. This was the Weapons and Tactics Instructor (WTI) course 2-21, at Marine Corps Air Ground Combat Center, in Twentynine Palms about 10km away.
Here is a promotional video from the exercise.
https://www.dvidshub.net/video/791782/wti-assault-support-training-4-b-roll