To your question about Kohein Gadol messing up on the avodah and dying on the spot, there is a gemoro in Yoma 9a
“The Second Temple stood for 420 years, ve-shimshu bo yoter mi-shelosh
meot Kohanim, and more than 300 served as High Priest over that
period. Take out forty years in which Shimon the Tzaddik was the High
Priest and deduct another 80 years in which Yochanan the Kohen Gadol
served, and ten more years that Elazar b. Charson served.” That
leaves 300 priests over 290 years. “Kol echad ve-echad lo hotzi
shenato, none of them lived out their year in office.”
Rabbi Shmuel Herzfeld says
When he entered the Holy of Holies, he literally put his life on the
line. If he made a mistake in the service, if he had a bad thought,
or if he was simply unworthy, he could instantly be struck dead from
heaven.
This, he explains, is why the Mishna in Yoma 70a says
ויום טוב היה עושה לאוהביו בשעה שיצא בשלום מן הקודש: “The High Priest
would make a party for all his loved ones when he entered the
Sanctuary in peace and left in peace without injury.”
About the second question:
See this article
which says what I have learnt
The Talmud teaches that the Beis HaMikdash was destroyed because of
baseless hatred. Bnei Yisrael did not properly fulfill the mitzvah to
“love your fellow as yourself.” Their strife and infighting caused
the Shechinah to depart from the Beis HaMikdash, and once that
happened, our enemies were successful.
The source is Sanhedrin 96b
A divine voice came forth from Heaven saying to Nevuzaradan "Thou
hast slain a dead people, thou hast burned a Temple already burned,
thou hast ground flour already ground.”
which means that the essence of the Beis HaMikdash was already destroyed when he destroyed it because the Shechinah had already departed.