Interestingly no Posner buffs have replied. That's because they probably haven't looked at the detail very closely.
To be sure, there are Posner folks on the web who support his work and have attempted to debunk critics. Some Posner critics, like other conspiracy kooks, go off on outlandish theories and say Posner had this or that motive to toe the lone gunman line. I doubt that's true. He simply found support for his theory in WC testimony while unilaterally ignoring testimony that didn't support his theory. A lot of people do that; they are called lawyers -- which is what Posner is. While he shouldn't be executed for doing that, his book shouldn't be taken terribly seriously. Its simply another lone gunman theory book with some terrible flaws -- the major ones pointed out by Weisberg unrefuted. Posner's kooky critics have been dealt with, and he would like you to believe that Weisberg has as well, but he hasn't. Additionally, Posner has been accused of fabricating an interview and refuses to turn over tapes of that interview he says he has. Search google for details.
Oswald DID NOT act alone. I don't know who did it, but the head shot clearly came from the front. Otherwise, the first shot to JFK's upper back would have caused some reaction like the head shot did. The only reaction it caused was some sort of probable involuntary movement of his arms and elbows in an upward manner. The first hit barely forced him forward in his seat -- he was leaning a bit forward and a bit to his left, but the fatal shot sent him violently over to his left. And for some reason, the gun that fired a bullet capable of going all the way through JFK's back and then through Connally's chest, leg, and arm, was suddenly UNABLE to fire a bullet that, once it shaved off the right side of a skull, stayed for the most part in the skull and did not go any further to hit Ms. Connaly or the driver -- or even into the seats in front of JFK. Nobody has even remotely come close to explaining that.
I don't think they can.