You say "the vision" Gabriel mentiones in Dan 9:23 is referring to the vision Daniel received about a decade earlier? Even though prior to the vision Daniel is now receiving he has uttered a very lengthy prayer, in which he not once mentioned the previous vision? And the vision he is receiving now is a clear answer of his prayer? So you see no way that Gabriel is simply talking about the vision he is about to give to Daniel?
I agree Daniel was not praying about the little horn vision. Nor was Daniel expecting Gabriel to appear to him in response to his prayer in Daniel 9.
But Gabriel did appear to Daniel over his prayer concerning the 70 years of exile about to be over.
Gabriel gave him the additional degree to the 70 years of there being the 70 weeks determined upon Jerusalem and Daniel's people and the Jews.
The reference to the Daniel 8 vision, in v23, and how it ties in with the 70 weeks, as additional explanation is because the last verse of Daniel 8....
27 And I Daniel fainted, and was sick certain days; afterward I rose up, and did the king's business; and I was astonished at the vision, but none understood it.
So Daniel did not totally understand it himself.
Daniel was told about the little horn person heading south and east toward the pleasant land Israel in Daniel 8:9 - and that the little horn person vision is for the time of the end. But Daniel was not told about confirming the covenant for 7 years by the little horn person.
And Daniel was not told in Daniel 8, about the little horn person being of the people who would destroy a future temple and Jerusalem. Which we know from history that it was by the Roman Empire. That the little horn person would thus be descended from the Romans. (that information is repeated in Revelation 17:10 of being descended from the Julio Claudians.)
So the little horn person will be the prince who shall come, from a geographic location west and north of Israel, from the geographic location of one the breakup kingdoms Greece. And he will confirm the covenant for 7 years.
In Daniel 9, the only covenant referred to that Daniel had prayed over was the Mt Sinai covenant, the law of Moses covenant. And as I had mentioned before, Moses had made a stipulation in Deuteronomy 31:9-13, that all future leaders of Israel on a 7year cycle have the law read to the nation, essentially confirming the Mt. Sinai that God gave the land of Israel to the children of Israel as theirs, forever.
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The little horn person, following Gog/Magog, enters into the middle east and Israel under the pretense of being peace keeper. The Jews will think he is the messiah, and he will be anointed the King of Israel by the false prophet (perceived by the Jews as Elijah) - making the little horn person the Antichrist.
As the Antichrist, leader of Israel, he will have the law read to the nation of Israel, from the temple mount. That is what actually begins the 7years of the 70th week.
Around 3 years into the 7 years - everything goes awry, with the Antichrist committing the transgression of desolation act of 2Thessalonians2:4, revealing himself as the man of sin - and not the messiah after all.
God in Ezekiel 28:1-10, has him assassinated by strangers for his audacious act. Then after his soul a few days in hell, returns his soul to his body - in disdain for the person - that he will be not long after be cast alive into the lake of fire.
Bringing the person back to life is the strong delusion God sends in 2Thessalonians2:11 to them who believe his initial lying claim of having achieved God-hood - that they should believe his lie.
Brought back to life, possessed by the spirit of the garden of Eden serpent beast, he becomes the beast of Revelation 13. The false prophet has a statue image made of him, and places it on the temple mount. The abomination of desolation. And the great tribulation begins. To end when Jesus returns.