In what way do your talking points support your argument? They don't.
It is known by everyone but perhaps you, that the dossier was started by an anti-trump Republican pac. A FISA warrant needs to meet a low bar ONLY. The warrant against page was NOT what led to the Mueller investigation. Two separate things.
What was the next sentence immediately proceeding your damning line, "KEY piece of evidence used?
"when the team first sought to pursue a FISA order for Page in August 2016, a decision was made by OGC, or, or both that more information was needed to support a probable cause finding that Page was an agent of a foreign power."
In other words, they were already watching him, already suspecting him of working with the Russians. The Dossier was just another corroborating piece of evidence, which by the way has been shown to NOT be all fake! Much of it has been corroborated with other evidence. Besides, a FISA warrant again, only needs to meet a low bar. THAT IS HOW OUR SYSTEM WORKS. If you don't like the system, change it.
"The 400-plus page report, released Monday, found that the FBI had ample evidence to open its investigation — despite allegations of political bias."
"Horowitz also reaffirmed that the so-called Steele dossier, a collection of partly unverified reports about then-candidate Trump, "had no impact" on the bureau's decision to open the investigation."
Should I repeat that again?
"Horowitz also reaffirmed that the so-called Steele dossier, a collection of partly unverified reports about then-candidate Trump, "had no impact" on the bureau's decision to open the investigation."
https://www.npr.org/2019/12/11/786323546/doj-inspector-general-testifies-on-fbi-probe-of-trump-campaign