What does this have to do with Pinochet's
During the coup he penned a letter of endorsement for the CDP.
To establish his political bona fides with the new dictatorship?
After the coup he defended its necessity and participated in the drafting of the new constitution.
Which means that he recognized that Pinochet did not defend the old one.
Your word games aside, he retained all of the main features of the 1925 constitution and incorporated them into the 1980 constitution that exists today.
So seven years without a constitution? Some defense.
It was necessary to convene a constitutional convention because of the damage Allende had already done to the 1925 document well before Pinochet ever arrived.
What damage did Allende do to 1925 that required its complete abrogation?
Criminality in office forfeits electoral legitimacy.
And it was in the Senate's power to determine this.
The clause in question actually damns Allende rather than Pinochet,
It damns them both.
Nor can it be construed as a usurpation of Allende's office, considering that Allende lost that office not by force or expulsion, but rather by his own hand when he chose to commit suicide rather than face trial for his crimes before the legislature and judiciary.
And where did Pinochet stand in the line of succession?
What a shock - back to the word games. You asked "Did he [Frei] say that before, during, or after Pinochet's dictatorship?" The answer was all three.
To establish his political bona fides with the new dictatorship?
Why would he need to do that? Frei had already been calling for Allende's ouster for months. Pinochet knew where he stood.
So seven years without a constitution?
Actually three, which is not surprising considering the amount of damage Allende did to constitutional government. There was a lot of sweeping up to do after him. Pinochet restored a provisional government in 1976 though to draft the 1980 constitution.
What damage did Allende do to 1925 that required its complete abrogation?
Ask him. He's the one that completely abrogated it. I suspect the answer would reflect the fact that he trampled the legislative and judicial branches while unconstitutionally seizing power into his own.
And it was in the Senate's power to determine this.
Actually it was the judiciary's power to decide criminality. The Senate's power was to decide the legislative trial (sorta like Chile's equivalent of impeachment). But Allende chose to off himself before he could be brought to justice.
It damns them both.
Repeat that all you want, but it simply isn't in the clause. Only seizing unlawful powers is, as Allende did. Arresting a criminal is not a seizure of power though.
And where did Pinochet stand in the line of succession?
I don't know what the exact Chilean succession statutes were at the time, but he would've been somewhere near the top of it as commander of the military. Pretty much everybody who would've been ahead of him - the political officers in Allende's regime - fled the country though, or got taken into custody for their own criminal acts. The succession eventually came to Pinochet because he was the highest ranked of the non-Allendists.