CClyde said:
There are two simultaneities to consider
There is only one that is relevant. The time of an event.
CClyde said:
, the emissions (two events as they have spatial separation) and the arrival of the light from those events at an observer (one event when simultaneous)
This is irrelevant, as not all events emit light and very few events can be "seen". A train timetable is based on where the train is and not when light from the train reaches you. For example, at train at Edinburgh train station cannot be seen from a London train station. There is no light signal from a departure event at Edinburgh to the destination station at London. And, yet, the event "train left Edinburgh on time at 13:45" is a well-defined event. Even if Edinburgh station was in pitch darkness.
CClyde said:
Assigning a time coordinate to the emission events is as Ibix said, a matter of convention. We cannot know our motion relative to the emission events,
There is no such thing as "motion relative to an event". An event is a point in spacetime.
CClyde said:
so we adopt a convention to set a time coordinate for the emissions. If they are the same, they are simultaneous.
But a convention such as the Einstein convention can only establish a time from observer to a source/reflector, not a time to a spacetime coordinate unless the two are the same which they would not be if the source frame is in motion relative to the event.
Again, there is no such thing as motion relative to an event. Note that your first fundamantal problem is that you do not properly understand the concept of spacetime.
CClyde said:
This is why the moving frame (sources included) finds no simultaneity in the arrival times of light from two simultaneous emissions
Simultaneity of events has nothing to do with light signals from those events. You can do physics in a dark room. You do not need electromagnetic radiation to have a coordinate system.
Some of the texts on SR over-emphasise the role of light signals. But, even if there were no such thing as EM radiation, SR would still be a valid theory of spacetime.
CClyde said:
This tells me there is a significance kinematic distinction between moving frames revealed in the simultaneity of light events.
Which is a fundamental misunderstanding on your part. Which, moreover, I doubt John Norton shares.
CClyde said:
Is there a flaw in this reasoning?
Yes. As above, it's all wrong.