If the "Shom'im" refers to the witnesses, who heard the blasphemy, why does the Torah add the word "Kol"?
Rashi and Targum Yonasan: "Kol" comes to include the Dayanim, who heard the blasphemy repeated by the witnesses.
What is the significance of placing their hands on the sinner's head?
Rashi (from Toras Kohanim): They place their hands on his head and announce 'Your blood is on your head! We will not be punished on account of your death, and you have only yourself to blame!' 1
Da'as Zekenim, Hadar Zekenim, both citing Toras Kohanim 2 : Witnesses about blasphemy must say [the curse] like they heard from the blasphemer. Moshav Zekenim - only one witness says the curse. (The others say 'I heard like him' - Sanhedrin 56a.) Therefore, they put their hands on him, and say 'this Aveirah that you cause us - it is on you!' Riva - this is like a Korban for all of them. He caused also those who heard to need to tear their clothes for hearing the curse. Gra (on Toras Kohanim) - also the judges hear it, from the witnesses. Through Semichah, they put the Aveirah on him, like we find in Semichah on the goat sent to Azazel!
This implies that the same applies to all who are executed. However, the Ramban, and R. Shimshon and Ra'avad on Toras Kohanim, say that it is only for Megadef. Moshav Zekenim citing Maharam says that we find laws particular to one Misah, e.g. an enticer and Ir ha'Nidachas. Also this can be different! Refer to 24:14:2:2. The Gemara did not discuss this Semichah. (PF)
Perhaps this is their Perush of Toras Kohanim; the text says 'you caused this to yourself.' Perhaps the other witnesses do Semichah just like the judges, for the blasphemer caused them to hear the curse. (PF)
What are the connotations of "Kol ha'Eidah"?
Rashi: It means that the stoning was performed in the presence of the entire community (whose Shelichus they were carrying out). 1
And it was as if three had all stoned him. Rashi: From which we learn the principle that 'The Shali'ach of a person is like himself '.
What do we learn from "Oso"?
Moshav Zekenim (like Chachamim, Sanhedrin 45a): He is stoned without his clothes, but not a woman (she is stoned clothed). 1
Chizkuni (like R. Yehudah, Sanhedrin 45a): He is stoned without his clothes (and the same applies to a woman).
Tosfos ha'Rosh (45a): We cannot make two Drashos from one word! It says "Oso" twice - in the command, and when they fulfilled it (verse 23). Tosfos (45a) - we do not expound about a woman. Chachamim decreed this, due to Hirhurim of men, or shame. Torah Temimah - We expound Oso, for it did not write Yirgemuhu, like above (20:2).