I am not going to try the case here. Don't be surprised if his legal team takes a different view.
Further, while your comments have some merit, they do not counter my point. Mannafort and Flynn each have their own story so you can't just put them both in a shake and bake bag and give the same reasoning. But in regard to Mannafort, there were two or three rounds of fisa warrants, and yes, the standards to get a fisa warrant are high and even higher for an American citizen, and yes, possibly/probably the were legitimately authorized (although I don't know, it requires the intelligence community to have been truthful with the court about what evidence they had and I don't automatically accept that).
However, the law is clear that the fisa warrant is only for the purpose of authorizing the collection of foreign intelligence- not for the purpose of collecting evidence of a crime. Obviously those two areas overlap because you can come across crimes. And where such and same occur with an American citizen you must immediately go to a normal Article 3 court and get a criminal warrant if you are treating them or have a reasonable expectation that they could become the target of a criminal investigation. That does not negate your fisa warrant for collection of foreign intelligence but neither does it let you use what you get as evidence- you need a criminal warrant for that at that point.
As you said, "he was a direct target." If they followed the law, then they are all set. If they just had a fisa warrant and are using that to do a criminal investigation. Then refer to my comments about their lawyers probably going to have more to say.
Not going to try it here but it is not a slam dunk yet for anyone. And Mueller is just trying to do shock and awe against mannafort - recall that he raided his home in the middle of the night- to get him to provide info on the trump campaign. It may work or it may not. It could be counterproductive too where his lawyers just say "fuck em, just plead the fifth, and we have perfectly good defenses." In fact that is the risk Mueller or any prosecutor runs. You can do shock and awe on one defendant in order to let everyone in the pipeline know that they need to get serious about cooperating but it can go the other way and a pantload of people who felt like they would just help out because they have not done anything wrong, suddenly just plead the fifth and tell the prosecutors to have a nice day.
We shall see.
Believer it or not, what you’re suggesting could mean manafort was actually operational for Russian intelligence rather than just colluding to benefit his client, Trump. Which would be a whole ‘nother bomb.
But, we will know more when we see the perfected charges (if it is Manafort being charged). I maintain his goose is cooked in the long run.