It sounds like this guy is pro se - the first thing he should do is consult with a great attorney that specializes in this area, even if it's only a consultation.
Based on the cites in your post I'm assuming the charges are only federal? Is it only 18 USC § 2252 (a)(2)(A), basically possession on the federal level? I think it might be easy to read the word "knowingly" in there and think the case is easier to win than it actually is.
In Edwards, he lost his appeal: "We AFFIRM the district court's denial of the motion to suppress based on the good-faith exception to the exclusionary rule, despite the lack of probable cause supporting the search warrant." I'm not sure in what way he would be using Edwards as part of his defense.
Note that Jacobson was very different from this case - he received legal material and later mounted an entrapment defense after 26 months of government attempts to entrap him.
About Caldwell, do you mean this one?: United States Court of Appeals,Fifth Circuit. UNITED STATES of America, Plaintiff-Appellee, v. Arkon Christopher CALDWELL, Defendant-Appellant. No. 08-50804. Decided: October 26, 2009
His defense being that he's a pedophile, likes to download child erotica and just accidentally downloaded CP comes across to me as incredibly risky. As in your first sentence, I think it would make it much harder for him to convince a jury that it was in fact accidental. Some other explanation I think would make the situation much easier to defend.
I don't know if the images/videos in question could be argued to be erotica, and not fitting the federal definition of CP. Or a search warrant/probable cause angle.
I am not a lawyer, but I think he really needs to start with a consultation - many charge nothing for an initial consultation, for a federal crime like this it's easy to find attorneys who will take the case, he should find the top names in his area, call them, and schedule consultations as a start. Even if I was an attorney I would probably still retain another attorney to argue the case and defend myself in court. Without a great deal of knowledge and experience in this specific area of legal defense he's basically guaranteeing that he's going in there half-cocked.