You make some very good points, to which I agree. I'll go into it in slightly more detail, trying to complement your findings. We know who it isn't:
- Kleo: for the obvious reasons;
- Ellen: she has several tattoos, some of which would be very visible in her exposed abdomen;
- Sonja: she has a more muscular build, which would also be reflected in her exposed abdomen.
Now, the girls who seem plausible:
- Venus: she appears to have a different "skin" texture, wider hips, and a different body shape in general than the girl present in the
mystery girl photo;
- Victoria: she appears to have a wider waist and hips and has an identifiable mark on her lower abdomen, but with certain light conditions, it's possible it wouldn't be noticeable in the
mystery girl photo;
- Shanlon: she also appears to have a wider waist and hips, as well as marks on her upper abdomen, but we could be fooled by an optical illusion due to the angle of the shot taken in the
mystery girl photo;
- Katie: she appears to have similar proportions to that of the girl in the
mystery girl photo, but she has a few small, but identifiable marks, similar (if not virtually identical) to Shanlon's, which aren't present in the said photo, although, it's possible that in the right light conditions, they wouldn't be noticeable there.
Now, the girls who seem probable:
- Abby: we don't have many examples to compare, but in the most favorable shots to proper observe her features (one which isn't very close, and the other where we can't see her lower abdomen) there doesn't appear to be any marks or details that could give her away, without mistake. But she appears to be slightly more toned;
- Chandra: she appears to have a similar waist, hips, and abdomen shape, but no further factors that could indicate if it's her or not;
- Gloria: she also appears to have a similar waist, hips, and abdomen shape, and she even appears to be wearing the very same lingerie set in one pic.
- Unknown girl: It's possible that it's a new girl, and not one we are already familiar with. One small detail that could hint that it's, in fact, a different girl is her belly button; just below it, there seems to be a small feature that seems unique, in contrast to all the other girls', but I suppose it could also be another illusion due to the lighting or angle of the shot.
Final verdict: If I would have to guess which of the known girls it is, I would bet in her being Gloria; the same conclusion my buddy
MacMan-88 reached, prior to my "investigation". If anyone find any contradictions in the points presented, or if you have found better examples and/or clues, feel free to present them.
Edit: corrected typo.