Click on these images for larger viewing.
After another day of continued debate and further scrutiny, I will offer one more item that I think will help settle this. Let's look once more at the Gardner image that Christopher Oakley suggests shows Lincoln, seated, in his top hat. My post yesterday points out that this "Lincoln" is seated behind and to the left of the center stage where it is reasonable to assume the POTUS would be seated. I have indicated Oakley's "Lincoln" with the big red "O". Move your eyes up and to the left. I have placed a white arrow pointing down to a man standing on a higher elevation, wearing what is a more casual slouch style hat, compared to those seated and to his front in top hats. Make note of the top hat figure directly to his front. Also note the higher figures to his rear and right.
Below, I have enlarged the slouch hat man a little more. His top hat wearing neighbor is partially cropped out of the view, to his left. Note a white shape that suggests an exposed shirt collar below the profile.
Moving on now to a detail from the image taken from the approximate front of the speakers' platform by another photographer. I have placed a white arrow pointing down toward an elevated figure in a slouch hat and a largely exposed white shirt collar. Partially concealing him in front is one of numerous men wearing ceremonial sashes. He has taken his hat off at this time. I suggest he is the same man in the previous Gardner view that was to the front left of the slouch hat man. Notice to the right, seated, is one of what is a cluster of men in top hats, more clearly seen in the fuller detail below this one. I am suggesting that this cluster of top hatted men is the grouping seen in the Gardner image where the "Lincoln" is supposed to be seated. By seeing these two images, known to be taken at approximately 60 degrees from each other, we can place the slightly elevated figures in relation to the seated figures and in the larger detail, see where Lincoln truly is seated, further off to the left and in what would be the center of the platform.
Below, the larger detail shows the cluster of men in top hats, off to the right, with our elevated friends rising to the left, and gradually sloping back down, all the while essentially forming a semi-circle around the President who is seated, bareheaded, to the left of center, above the heads of the audience in front.
And in closing, to supply a few requests for some indication of the camera positions in relation to the speakers' platform, here is an aerial view with red lines from each camera position pointing directly to the platform. The yellow line indicates the approximate 78 yard separation between the two photographers.
The camera positions are placed according to the map created by William Frassanito in his book, Early Photography at Gettysburg, page 139, and are approximates.