Shlichus by Mitzvos

The Ktzos HaChoshen in Siman 182 discusses why it is that shlichus doesn't work by mitzvos. For example, I can't tell my friend to put on tefillin for me as a shliach. Why not? The Ktzos explains that mitzvos are specifically instructed to be b'gufo, I must put the tefillin on my arm. One question on the Ktzos is from the psak of the Rema in Siman 322, that a father cannot appoint a shliach to do the mitzvas milah on his son for him. Why not? Here the mitzvah is not b'gufo?
Rav Chaim (stencils hosafa 5) suggested a different approach. He suggested that shlichus only works on areas of halacha where the mishaleach is a baalim on the dvar hashlichus. As we know, the sources for shlichus come from gittin, kiddushin, and terumah. In all these cases the mishaleach has baalus. For example, if a man appoints a shliach to carry out a kiddushin for him. This is an area of halacha that involves baalus & kinyanim. Because the man is a baalim on his kiddushin, he can appoint a shliach to do the maaseh kiddushin for him.
Evidence to this can be found in the gemara in Kesuvos 74a where it says that only in areas of halacha where one can appoint a shliach, can one make a tnai. The idea according to Rav Chaim is that in both situations, tnai and shlichus, one is demonstrating baalus over the action taking place. (See sefer Eretz HaTzvi by Rav Schachter who brings further proof from Tosafos in that gemara.)
From this logic stems a number of chiddushim regarding a shliach l'kabbalah by a get. We know that by divorce the husband may appoint a shliach to deliver the get on his behalf. This is called a shliach l'holacha. Then, there is another halacha that a woman can appoint a shliach to accept the get on her behalf. This is a shliach l'kabbalah. The Tosafos HaRosh in the beginning of the second perek of Kiddushin asks, why do we need a separate pasuk for shliach l'kabbalah? The answer given is that we know that a woman can be divorced against her will. This is evidence of the fact that a woman cannot exercise any daas when it comes to divorce. If the woman has no daas in this area, it would seem that she lacks baalus, in which case she should not be able to appoint a shliach. This is why a special pasuk is needed to include shliach l'kabbalah. Otherwise such an idea would have been impossible, due to the lack of baalus.
Another famous area of halacha that daas comes up in, is within the issue of eidus l'kiyum hadavar. In normal situations, witnesses are used l'vrurei milsah, to clarify the matter. We use eidim to determine what precisely happened. However, when it comes to gittin and kiddushin, there is another type of eidus, eidus l'kyumei milsah. Witnesses must be present at a marriage and divorce not just to confirm that there was a marriage and divorce, but even to allow the marriage or divorce to take place. Without the eidim, the kiddushin/gerushin cannot even be chal. Rav Chaim in Yibbum V'Chalitza 4:16 points out that by chalitza, eidim are not needed l'kyumei milsah (at least not min hatorah). Why not? He explains that eidim l'kyumei milsah are only needed for davar shebervah that involves daas. Kiddushin and Gerushin involve daas. This is not the case by chalitza. We also find that specifically by shliach l'kabbalah, and not shliach l'holacha, eidim l'kiyum hadavar are needed. It seems that specifically when appointing shliach l'kabbalah, there is "daas davar shebervah" that doesn't exist by shliach l'holacha. This, Rav Chaim discusses in Gerushin 6:9.