Eudora Welty's "A Worn Path" does present Phoenix, specifically, and others like her, generally, in the grip of large and indifferent social and political forces. This is a Southern Gothic story that deals with life in the South after the Civil War. For the most part, existence was not good for anyone immediately after the Civil War. The South was ravaged (remember most of the battles took place in the South) and the economic system turned upside down.
Notice that in the story no one is really out to get Phoenix. Her existence is the way it is, period. Nobody conspired to make her grandson swallow lye, or arranged things so that she would have to make such a long, difficult walk to get his medicine. Even the man who points the rifle at her does it just for the fun of it, for kicks. No one is persecuting Phoenix.
Yet, no one is really doing anything to help her, either. She has to suffer humiliation just to get the medicine for the grandson. And even this act of charity is done with indifference:
"A charity case, I suppose," said an attendant who sat at the desk before her."
Phoenix is the victim of society. Society is not necessarily against her, but it is not for her, either.
You could say that Phoenix faces a great number of obstacles. Some of them include:
- The thorns that catch her dress.
- The log that she has to use to cross the stream.
- The white man who is out hunting.
- The fact that her shoes are untied and she can't tie them.
- The animosity of the woman at the doctor's office.
In general, she is having to struggle against her own frailty and against the racism of the time.
For your second question, I think that it is poverty and racism that have her in their grip. You could read the whole journey as symbolic of her struggle against poverty (if she were well-off she wouldn't have such an arduous journey, I wouldn't think). You can also see racism at play in the way the hunter treats her and in the way the attendant treats her. In both cases, it's not really personal -- they just don't see her as a real person.