Having already warned (in the previous Pasuk) against touching the mountain, why does the Torah see fit to repeat it?
Rashbam, Da'as Zekenim citing R. Yitzchak, Hadar Zekenim, Rosh (12): The Torah is referring (not to touching the mountain, but) to touching the sinner. One may not come close to him to kill him, for then the executioner will be Chayav Misah for ascending the mountain! Rather, one stones him via firing arrows or throwing stones at him, as the Pasuk goes on to explain.
Moshav Zekenim: This verse forbids touching (killing) one who touched the mountain. We must let Shamayim kill him.
Da'as Zekenim: This verse teaches that he is stoned. Do not say that Stam Misah is choking.
What did Hashem mean when he added "O Yaroh Yiyareh"?
Rashi: This teaches us that the sentence of Sekilah was carried out by pushing the culprit down 1 from the place of Sekilah.
What is "bi'Mshoch ha'Yovel" that permits to ascend the mountain?
Rashi #2 (Beitzah, 5b, citing the Mechilta) and Rashbam: It is when the blowing of the Shofar ceases.
Rashi in Beitzah, 5b. It is like the way of musicians to end with a long note. (This is like we do in Teki'as Shofar on Rosh Hashanah
What exactly was the "Yovel"?
Rashi: It was the Shofar that was made from the horn of Yitzchak's ram. 1
Ramban (according to Kabalah): It was the voice of Pachad Yitzchak. 2
Rashi writes that "Oh Yaro Yiyareh" teaches that one who is stoned is cast down. It says "Sakol Yisakel" before this. Beis Din casts him down before throwing stones! Within one matter, the Torah is in order (Pesachim 6b)!
Tosfos (Pesachim 6b DH Aval), Moshav Zekenim #2, Riva: Here is different, for a Drashah teaches that stoning is [written] next to death - "u'Skaltam ba'Avanim va'Mesu" (Devarim 17:5). Therefore, stoning is after casting down.
Riva, citing R. Meir Kochbi, Hadar Zekenim: The Torah needed to write Oh Yaro Yiyareh last, to teach that if he died from the fall, they were Yotzei (without stoning him). There is no need to teach that if he died through stoning, they were Yotzei.
The verse does not discuss Misas Beis Din. Refer to 19:13:152:4.
Rashi writes that "Oh Yaro Yiyareh" teaches that one who is stoned is cast down. Why does it say "Oh"? In every case he is cast down (and if this does not kill him, they stone him)!
Riva: We find that "Oh" can mean 'Asher' (that), e.g. "v'Higisho El ha'Deles Oh El ha'Mezuzah" (below, 21:6). The Targum Yerushalmi is 'to the door that is attached to the Mezuzah.' Here it means that we stone one who was cast down.
Refer to 19:13:151:3.
Hadar Zekenim: Here, "Oh" means Im (if)