(1) As author Zorn and his crime experts agree, it is highly unlikely that the kidnapping - if that's what indeed happened (remember that there have been other theories) - could have been carried out by a single individual. Most feel it would have required three men, which is Zorn's theory. So if Hauptmann did it, he didn't do it alone.
(2) There was hardly any evidence placing Hauptmann at the Lindbergh estate on the night the kidnapping was supposed to have occurred. There was one elderly man with cataracts who had severe visual problems who claimed he saw Hauptmann in the vicinity. Not only was he virtually blind but he was paid off for his testimony. The best point the prosecution made in that regard was a so-called government "wood expert" alleging that a piece of a ladder found at the Lindbergh property matched part of a floorboard found in Hauptmann's attic. Of course, back then, no one would question the honesty and integrity of a government bureaucrat. But no one from the defense team was in Hauptmann's house at the time that matching piece of Wood was supposedly taken.
(3) There is no doubt that Hauptmann was caught passing a bunch of ransom bills and had possession of $14,000 of the total $50,000 in ransom currency. But that makes a good case for extortion, not kidnapping-murder. He could have been an accessory after the fact as part of an extortion gang that took advantage of the well-publicized purported kidnapping. Or he could have purchased that ransom currency at a discount at some point down a chain after it was turned over to "Cemetery John." Any of those scenarios would make him guilty, but of lesser crimes.
A defendant's demeanor shouldn't mean anything to an objective juror. Whether guilty or innocent, it usually is very difficult for any defendant to bear up well against cross-examination by the prosecution. He would generally be anxious or even angry regardless. That's why defense attorneys are usually making a mistake by letting the defendant testify on his own behalf. That's a pretty standard rule these days for the defense. Yet Hauptmann's attorneys back then were foolish enough to put him on the stand.
The controversy over Hauptman started,in the main, with that vacuous being Eleanor Roosevelt who wrote a blurb criticising the fact that circumstantial evidence was used to convict Hauptman.
Good circumstantial evidence is far better than eye witnesses as circumstantial evidence never lies.(Indeed, you claim one of the "eyewitnesses" was near blind and received money.) Once again, as far as man can be sure of anything on this earth, Bruno Hauptman was guilty of the kidnapping and murder of the child.
You liked the book and that is ok, I would point out to you another who dunnit involving the unsolved murder of Hollywood movie director William Desmond Taylor in February 1922. Do a Google search Using Taylorology. Also, WIKI has a good summary under his name of William Desmond Taylor(Not his real name.)