Doc,
A most interesting post indeed....
If I am understanding you correctly the horizontal punch should only be used when it is at shoulder level quote]
I believe it is shoulder level or below. Many of these if you pay attention, your body will tell you the answer. Example; an outward or back elbow to the head should be executed with your palm down, while solar plexuses and below the elbow should be executed with the palm of your hand facing up. Try reversing the direction of your palm and if you listen to your body you will feel your muscles bind up, ex- if you were to do an outward elbow to the head with your palm up.
As mentioned it also has to do with the target you are hitting. While many weapons may work, the smallest weapon that fits the intended target will give you the greatest concentration of impact with the greatest results.
Take for example a side kick to the groin and for the sake of argument lets just say the the opponent is facing you square, straight on. A well executed side kick might just happen to work effectiviely. However; it is possible that with the foot sideways the force may dissipate over a greater area or the heel may hit off to the side of one thigh and the toe end of the foot may make contact with the other thigh killing a lot of the intended impact to the groin.
So for this example, a front instep kick or a back kick will fit nicely into the groin target hitting the target right on with concentrated impact producing maximum results. This is an example of weapons fitting the target better.
The verticle punch fits into the solar plex target tighter than a straight punch [horizontal fist].
I am in complete agreement with Doc's explanation too.
Yours in Kenpo, Teej