Scourge of War Boards

Was it likely that the shorter musket ranges caused the Guard not to form square as often as they should?

Indirectly, because the stance is set at musket range, but if the stance is already set (for the above cases) there's a check to 250 yards for the nearest enemy

...and is it possible to disconnect that link between musket range and reaction distance?

You can use enemybeg() but how much this can impact on the performance repeated every loop I don't know.

One thing this might allow to be represented is if you can set a reaction distance longer for poorer quality troops so as to represent them forming square early out of fear or lack of good officers, etc. A cavalry threat could then pin poorer quality troops into square from further away, making them less "reliable".

There's something similar but stance related; in teh AI you cannot access directly the skills point but read the values of unitattributes.csv, so you can link this distance to some columns logically similar like wheeling locked