4) there was a FISA warrant but its limits and/or targets were exceeded.
5) There may have been a warrant that did not reference Trump, Trump Tower, or possibly even anyone in Trump’s circle.... i.e., a warrant for surveillance on one or more Russian banks or persons. It could well be that someone in the intel agencies exceeded the lawful limits, it’s not like that has never happened in history!
6) the warrant(s) is(are) legally valid, but is(are) based on invalid data, and/or invalid analysis based on that data
To me, it would make the most sense if most of the stuff being argued about was slipped over the heads of the legal folks (including fisa judges) and political staffers via an “expert” analysis presented to the fisa judges.
it only takes one technical “expert” to testify, with no alternative analysis required to be presented by the non-adversarial proceeding, for the entire process to be corrupted. if my understanding of the fisa proceeding is more or less correct(?), this could be the achilles’ heel of the entire process.
obama and his lieutenants would naturally gravitate to an achilles’ heel of any such proceeding in order to subvert it, i.e., in order to get the ends to justify the means.
it all depends on the definition of the word “valid”...