As you found out yourself, the subsequent images are used to display the eye and mouth movement. These are overlays over the original image. So you have a face and then images of slightly different mouths and eyes. These are then drawn over the face.

If I recall correctly,TableData/IMPPortraits.xml tells the game the offsets it should use for this. For example,

0
200
0
8
5
8
21
3
0
7
2
1

tells the game to use any 'eye'-pictures it has with an offset of 8/5 over the original face. In your case, this will copy over the same image then... I think setting the offsets to 0/0 should help you.

Nope, the 1.13 one should be enough...

What happens if you, say, overwrite the data of slot 18/218.sti with your picture instead?

Well, there is a good reason why the eyes and mouth animations are stored in smaller pieces. The animation technique used by JA2 for the faces is called dirty rectangle animation. Your approach of using a subsurface with the same size as the original portrait goes against the very concept of dirty rectangle animation and several problems will inevitably arise.

As you correctly pointed out, the subsurfaces of the eyes and mouth are blited over at special coordinates in order to show up at the right place. If you insert a subsurface with the dimension of the portrait, that subsurface will be blited to the lower right, unless the coordinates are set to (0,0). Setting the eye coordinates to (0,0) is possible, but then all subsurfaces for the eyes have to use those coordinates as well.
You can not however use (0,0) for the mouth coordinates. The mouth coordinates should always have a higher y-value than the y-value of the eyes plus the height of the eyes subsurfaces. Setting the y-value of the mouth to zero will inevitably blit over the eye animation and therefore interfere with it.