Sorry, I have been somewhat AWOL myself with some unexpected travel'
That is more or less the question. Or, rather, one of how to integrate various features of SFTypes that are both realistic and make sense in terms of a a game. Your point about the function of artillery versus mortars and infantry guns is a good one, and one that I was effectively wrestling with. It was one reason that I combined machineguns with mortars to make a heavy weapons unit. The same goes for infantry guns and artillery. If artillery has no range, why bother with infantry guns. They function the same, just a difference in stats and costs, and, frankly, the player would be better off doing a cost analysis to see where they got the biggest bang for their buck.
Also, it does seem as if some infantry regiments did bypass the infantry guns for actual artillery in their artillery battalions, namely 105 mm or 155 mm howitzers. From the standpoint of building a scenario this all fine and dandy, but with a game where you "build" your units, game design would give of these SFTypes a purpose, i.e., a unique function that makes them worth using.
I believe there are two design options.
The first is to combine the machineguns and mortars into a heavy weapons unit and also combine infantry guns and artillery into an artillery support unit, with perhaps a range of one hex. They would be embedded in the divisions, and artillery barrages wouldn't be part of the game. This would be the most realistic, but perhaps the least fun? And, would eliminate an aspect where artillery is used to affect the readiness of an intended attack target. This would seem to have to be the proper option for a corps level game with 40-70 miles/hex and one month turns.
The second, which I am somewhat inclined to for a division level 15 mi/hex, two week turn, is to keep a ranged artillery unit, with an unrealistic range of two hexes. One reason is to some specialization of units as part of the game. The second is that the ATG engine is set up to utilize artillery. The AI makes use of it, and it is an aspect, like I said, where you can use the artillery as part of the overall attack strategy, both on attack and defense. The two hex range also allows for better stacking and the need to "find" the artillery. Otherwise, one would have dueling artillery all the time.
Artillery as designed with the ATG engine is probably more realistic for a regimental scale (i.e., DC 1 & DC2) of say 5 mi/hex a few days/turn. Although, I would say that one this scale naval engagements are rather odd.