It is hard to say whether the upper shot is a Southern Small White from this photo, but the one visible clue suggests not, at least to me. The unf black spot usually has a straight or concave external edge for Southern, so this looks like a Small White to me. I think this is probably a consistent feature. A shot of the upf apical mark would help enormously if you have one. The extent to which the upf apical mark extends along the margin is usually reliable - Southern extends down to vein 3 or 4 and Small only to vein 5 or 6. I think/hope this photo illustrates this point.
The distribution of the grey scales on the unh is also a good indicator - for Small the grey scales are mainly below the cell and for Southern, usually even across the unh. There is some clue regarding whether the fw vein 7 is forked, but I have no real experience of this.
The top photo looks to me like Small to me but I'm inclined to agree with you that the lower photo could well be Southern. I would probably want something more conclusive to put it in my life tick list, though.
However, I'm in the UK at the moment and my books are in France, so I'm working from what may be a fallible memory.
I'd be interested in the views on the caterpillar - here is a shot I took last month which looks to be the same thing.