1. I don't think that if Dorthea would've used photoshop or known it was around (not that it was) but if so, she probably wouldn't have taken so much time taking her pictures and getting the perfect light, contrast, etc, if she knew she could've just went in and changed whatever after she took the pictures.
2. No, her meanings came across very clearly in her original pictures.
3. Well yeah if her photos were all just faked then they wouldn't have had as much meaning and they probably wouldn't look as good either.