Mel could do any or all of those things. I totally agree that security would be the biggest issue - though I agreee that he might and could film certain things in secret.
There are many things which are filmed quietly - documentery interviews.......film student projects.....are usually made so they do not attract attention.
What few realize is that technology has made possible the filming of people and conversations in a very discreet manner. Camcorders are very small now......and cell phones can record images as well. One can easily edit things like this together.
Who is to say that he has not filmed certain things for such a venture? He has the $$$ and resources to do this.
The thing which most have forgotten is that in the early 90s, Mel Gibson had a very private audience with the Pope. The exact subject matter was never really disclosed. But i do recall reading in the press that he aid something to the extent that the Holy Father had wanted him to do something with his talent and abilities/resources.
Malachi Martin disclosed that he had been given a special permission from the pope (I think he meant Paul VI) to be released from his vow of poverty so he could live on his own, but that since he had read the 3rd Secret, he could no longer in good conscience say the novus ordo mess. He was sworn to not divulge the contents of the Secret. He also said there are then 700 priests like him...
You bring up some interesting points. Mel has the bucks now to accomplish a lot of things. We know he has a great desire to do so. He also is in a position to have access to information that most of us can't get, so he would have to be very discreet in what the film portrays. He could, for example proceed as if certain rumors are true, but run the risk that those rumors could be false or misleading.
The "trail of breadcrumbs" you say is a curious concept. Since there are so many loose ends regarding Fatima, Mel could really develop that aspect by leaving numerous loose ends in the film, a sort of artistic open-endedness to imitate the topic being depicted.
As for the contents of the 3rd Secret, for example, he could touch on all the false editions that have been published, and all the reactions to them, including how their respective followings have died off after the contradictions have worn out the claim.
It would almost be too easy to make the Vatican look silly in what Ratzinger et. al. have said, given the facts. There is certianly enough material just in this entrigue to make a movie, so he would have to be judicious if he wanted to avoid letting that aspect take over the whole film. But if he did it right, and we know he's capable, it would put the onus on the pope to make the consecration at last, because the deficiencies of previous consecrations would be plain for all to see. Imagine how furious people like Fr. Fox would be to see his pet theories being exposed for their ridiculousness! He would probably try to sue Gibson for defamation or whatever. So Mel would have to contend with that kind of issue as well. The Fatima story is current events, whereas the Passion was practically speaking, in large an ancient issue.
I know a man who is Protestant and he rents a house from a Catholic relative. The relative has a shrine to Our Lady of Grace in the side yard of the rental house, and the Prot can't avoid seeing it when he comes and goes, which makes him miserable. The really wierd part of this is, that he has a son named "Elijia" and he has very strong convictions about the prophesy that Elias will return before the Second Coming. I recently read a transcript from a speech by Gerry Matatics that proposes that Our Lady is the fulfillment of the Elias prophesy, and that at Fatima she gave the most clear fulfillment of that prophesy. I am wondering what my Prot friend will have to say about that.