Very interesting questions. Good for you for taking the plunge and trying out a CGM. One of the runners I coach, Luke Nelson was part of a study group testing CGMs on endurance athletes. I can ask Luke to chime in with what he learned.
I think some of you suppositions are very likely to be correct.
Spikes in glucose doing intensity: My wife who is a Type 1 diabetic sees big spike in blood sugar doing intensity. That can have significant negative health consequences for Type ones so I try to discourage that type of activity.
Low glucose levels probably indicates that fat is the dominant fuel source.
It makes intuitive sense to me that this would correlate well with the aerobic threshold. Once above ones AeT you start recruiting more and more FT fibers that require more glucose for fuel.
We just an endocrinologist that’s an ultra runner to come in here and set the record straight.