The Trustworthiness of FISA Warrants

If the claims of the now famous HPSCI "Memo" are accurate, the DOJ/FBI used campaign opposition dirt on the Trump campaign and a DNC/Clinton smear campaign to get a FISA warrant to spy on at least one American citizen. Anyone involved in that decision, if they are still working in our government, should be fired. And if it indeed does turn out, as it currently appears, that the origin and nature of the dossier and its contents were withheld from the FISA Judge then everyone involved in that decision should probably be indicted and prosecuted.

Since there seems to be no real check on what is presented as evidence to the FISA court when seeking a warrant, how does the use of this dossier not call into question the validity of every FISA warrant this secret court has ever issued?

What safeguards are in place to prevent this kind of disturbing abuse from happening again? How many times has it happened already? The mechanisms to do this to Carter Page (and who knows who else yet) were clearly already well established and in place. Every FISA warrant granted during the Obama Administration should now be immediately reviewed to see if this has been done before. I'd even prefer they go back and review every FISA warrant issued since the whole War on Terror began after September 11th.

So far, it appears as though Carter Page hasn't broken any law and seems to be under no threat of indictment. So, what was the FISA warrant even for? Even more importantly, why was it renewed 3 times? Were they not interested in Page at all, but dangling him out there as bait? Who did they really have their eyes on?