Would there be a "fragment trail" from a full metal copper jacket?
Yes, but it would be nothing like the high fragment trail, which includes dozens of tiny fragments clustered toward the right-front part of the skull. FMJ bullets that strike skulls can leave fragments, but only a few. The FMJ bullets in the WC's wound ballistics tests left very few fragments, and none of them left anything resembling the cloud of fragments that compose most of the high fragment trail.
We simply don't know how many fragments were in the low fragment trail, because it no longer appears on the skull x-rays, and because Humes did not mention how many fragments it appeared to contain. He said there were "multiple minute metallic fragments" in the low fragment trail, which he said ran along a line "joining the above described small occipital wound and the right supra-orbital ridge," i.e., just above right eye. "Multiple" can mean a few more than one or many more than one. "Minute," however, implies that the fragments were tiny, which is not typical of FMJ ammo.
Humes faced an obvious and serious problem: the x-rays showed two separate fragment trails, one low and another at least 2 inches higher. One of the trails had to go.
And it goes without saying that Humes could not have failed to see the high fragment trail, which starts/ends in the right frontal region with the cloud of fragments and dissipates considerably as it trails upward toward the back of the skull but does not reach the cowlick. This suggests the impact of a high-velocity frangible bullet in the right temple, and we have several witnesses in two different locations who saw a small wound in the right temple.
The plotters had to pick their poison. The EOP entry site presented an impossible trajectory back to the sixth-floor window, unless one assumes that JFK was leaning forward by about 60 degrees when the bullet struck, which no video or photo shows him doing at the time of the Z-film head shot. So the plotters decided to make the low fragment trail disappear and to plant evidence on the x-rays that would appear to indicate that the bullet struck about 4 inches above the EOP and that would superficially seem to be the source of the high fragment trail.
But, as is well known, there is no wound in the cowlick, and the high fragment trail does not extend to the cowlick. The cowlick entry point poses a much less severe trajectory problem than the EOP site, but that's not saying a whole lot. It's much less severe because the EOP site is self-evidently impossible to align back to the sixth-floor window. However, NASA's Thomas Canning, the HSCA's trajectory expert, had a hard time aligning the sixth-floor window with the trajectory of the cowlick entry site through to the supposed exit wound above the right ear.