Although it may be a little different, I work in the nonprofit sector as a fundraiser – first in arts & culture and now for a social organization that focuses on gender equality/girls eduction. I got my masters in nonprofit management/arts admin a few years ago and I would recommend the second option, accepting the job offer and trying to do the grad program PT. I've seen so many of my friends take full-time jobs while going to grad school and I also took on like two internships every semester, so basically was working everyday. The fact that most are evening classes makes this doable. Also, it sounds like you are invested in this field of work, so I think grad school is a great next step for you to deepen your expertise.
However, this depends on how much you want this specific job too, not just because it fits your interests but does this organization carry a mission you feel strongly about? do you see yourself as a good fit to its work culture? do you think you will like the people you work with? etc. These questions are especially important for nonprofit jobs because as you know, you don't get paid a lot. We are not doing it for the $, because if we were, we wouldn't have chosen this path in the first place!
I don't know what level this position you were offered is and where it's based in (which will change whether $55k salary is a great offer or not) but if we say it's an entry level position at a nonprofit in NYC, $55k is pretty good and I'd say accept it and try it out. If you happen to not like it, then you can always leave and find another nonprofit to work for – they are always looking for people.
That's my two cents. Hope it helps!