Ive yet to play but have found from other games that it depends on what players are used to as many tend to adopt their own preferred method as house rules rather than go by what is written.
given the examples above, general melee assumes that both groups are intermingled and would go for the whole group vs the whole group with casualties being taken from the rear as this is generally the easiest place to remove models than from in the middle of the fight. This works well if fighting in open terrain.
if fighting across an obstacle or in between 2 buildings, or on a bridge for example the unit gets bottled up by the confines of the terrain, in this case only the front ranks get to fight, with the exception that you could include second rank and file if they have spears or bole weapons being able to reach over or between combatants.
though to be fair i like to keep it quick and simple, count all models in melee, role for results remove casualties for either side. then you decide if is a draw or who is forced back or routed but that’s another argument.
the way i read it, is that chain shot would be a -2 unless fired high making it a -1. french only count -1 when firing high instead of the -2. so french firing high with chain shot overall modifier -2, everyone else -4… -3 for english with their +1 for gunnery
if using carronades.. range is halved from 8 to 4 inches but you count +2 if you shoot at pointblank range of 3 inches or less. so get in that little bit closer and then you suffer no minus modifier. it does mean though that you would take a battering from the English guns getting in close and then double damage at pointblank range.
would save the chain shot for ships with heavier guns. heavy cannons halved 10″ range light cannons 7″ and keep the carronades for throwing lumps of iron and then grape at point blank before boarding.