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Technique for independent, continuously variable shadow arrowhead size. Try to say that 10X fast!!!

It is not necessary to do the shadow arrowhead to be 1 size smaller than the normal arrowhead. That approach was too limited.
Rather, do the following.
1. In the group shape, add one more shadow line and send it to the back.
a. The line weight of this 2nd line will not be a fixed multiple of the original line weight. You'll either change it manually, or, you could create a shape data property at the group level, whose value you can readily change. Note, this shadow line weight will always be less than the original shadow weight, but more than the normal line.
2. In the shapesheet for the original shadow line, use the guard function to lock the line ends to none, i.e., guard(0).
3. Since the arrowhead scales with the lineweight, as you change the 2nd shadow line, you will change the amount of overlap of the arrowhead shadow. You can now finely tune the amount of overlap.
Summarizing, you have 3 stacked lines: top level is the normal level, middle level is the line shadow without any applied ends, and the bottom level which has line ends, but, its lineweight is independently adjustable -- smaller than the mid level line weight, but, slightly greater than the top level normal line weight. Thus the line shadow overlap will appear to remain constant, but the arrowhead shadow overlap is freely adjustable.
You can make this width track by the top level line width by guarded formula, and make the amount of overlap setable at the group level with an editable group level shape property.
Wapperdude