Suppose the firing mechanism was on the opposite side of her nightdress?
It doesn't matter, the cloud of PGSR forms under, above and to both sides of the mechanism.
If you are firing a rifle standing with the rifle eye level and could see the cloud as it expands it would be approximately a half foot above your head, down to your stomach area, around a foot left of your left shoulder and 1.5-2 feet of your right shoulder. Also a foot from your chest and foot from your back.
That is just the initial cloud, the particles can and will travel beyond this but will not be as concentrated as they will in the cloud that forms. The high concentration matters in trying to assess who a shooter was as opposed to incidental particles.
Her gown would have had GSR if she had stuck the gun under her neck and hugged the weapon regardless of whether the trigger guard were facing her body or away from it. For none to get on her gown while doing so would be incredible. Even if there were not enough to say she were definitely the shoorter there would have to be some.
A majority of the time today they are able to actually figure out how a gun was being held by a shooter who commits suicide based on the gunshot residue pattern on the hand. This is because victims do not move after dead so we don't have the kinds of spoilage of evidence that happens when a shooter is alive. Today we know a gread deal more about patterns of blood, GSR and other things than ever known in the past.
Sometimes this helps the defense other times it helps the prosecution, it is a double edged sword. Obviously it helps an innocent defendant much more often than a guilty one.