I did put up the Zappruder film, post #80 if you want to take a look at it.
Doing some quick research on this, it's been a while but according to witnesses Kennedy's occipital bone was shattered, which is in the back of the head. That's been disputed and now it's claimed it was the parietal bone, indicating a back hit.
There's also some dispute that there may have been two brain examinations, the second one that confirms what you're saying about the small entrance hole in the back. Here's the link to that which contains the following quote about the first examination:
"Horne contends that the damage to the second brain reflected a shot from behind. He says the first brain was Kennedy's and reflected a shot from the front."
So there's conflicting versions of events from supposedly credible sources. But the witnesses who claimed the occipital bone was shattered like Dr. Harper and Dr. Cairns said that from the very beginnning, and those are the ones I tend to believe.
The fragment of skull retrieved [Harper fragment] was occipital.
The physicians and nurses at Parkland Memorial reported a large wound of exit in the occipital region, and a small wound of entry in the throat.
The x-rays were tampered with (Assassination Science, ed. James H. Fetzer, Ph.D, Catfeet, 1998).
Conspiracy of Silence, Charles A. Crenshaw, M.C., Penguin/Signet, 1992, collates the testimony of the Parkland staff.
The autopsy at Bethesda is discussed in detail in Best Evidence, David Lifton, Dell, 1980.
Gerald Ford claimed the rear shot was in the neck, but the FBI measured the holes in the suitcoat at 5-3/8" from the collar and in the shirt at 5-3/4" from the collar, making a throat exit wound impossible--just as the Parkland staff said.
Throat shot from front; Oswald in back; QED Warren Commission wrong: Oswald not sole actor.