Hello,
CockneyRebel, I'm very sorry about your situation. I've worked for some nasty bosses, and unfortunately women can be just as abusive - sometimes in different ways - as men.
It helps very much when you explain your situation in enough detail so we can understand your situation. Please remember that most people reading this have no idea who you are or what you do. (When I say "you" here, I mean people in general.)
On the one hand, you, CockneyRebel, are certainly finishing your work on time. On the other hand, quite possibly your boss (1) wants you to go on to some other task when you're finished and/or (2) doesn't feel like paying you for what she (or her bosses) might consider an unnecessary five minutes. Remember that your time is only your own if no one's paying you for it.
That said, given your leg and ankle injuries some accommodation may be in order. That may qualify as a disability, at least with respect to how long you're allowed to take doing your job. In other words, you may be able to make a case that if you do the same job as anyone else, with no more help than anyone else, you should be allowed to take a few minutes more and be treated the same as your workmates do when they finish on time.
As for going back to the job, I suspect that ship sailed when you called her and told her you quit. In general, you really can't quit - especially after a dispute with your boss - and then expect to come back as if nothing had happened.
Also, there is a protocol to quitting. If you were as calm as you said here that you were*, that's good. Even better, and generally the norm, would be to give two weeks' notice, to give her time to find someone else and to adjust people's schedules.
I don't know what role your Job Placement Specialist has in this. If s/he's willing and happy to help you just the same no matter what happens with this job, that's good. However, if s/he's in a position to order you to go back to the job, and/or cut or withhild some kind of benefits, then you're in a bad situation and you should have consulted with him/her before quitting.
Incidentally, even if your boss has a boss of his/her own, complaining may do little good. Jack Welch (who served as CEO of General Electric for 20 years) advises against it, because most often either the second-level boss just ignores your complaint or scolds your first-level boss - who then retaliates against you. My experience also is that few higher-level bosses are willing to intervene on behalf of lower-level workers...as opposed to customers. (For that matter, higher-level bosses tend not to even care how their subordinates treat applicants.)
Bottom line: At this point, I'd see if the Job Placement Specialist will let you walk away from this cleanly, including without meeting with your supervisor. If that's not in the cards, be as professional as you can possibly be (not just as professional as you think the supervisor or the situation may deserve). If you can't persuade your Job Placement Specialist to at least not make you go back there, offer to work there for two weeks more since it's only fair to give that much notice when quitting a job.
Also, see if the supervisor will write you a letter of recommendation; if she puts good things about you in writing, she'll be much less credible if she tries to bad-mouth you later. If she refuses the letter, you at least know you may have trouble, reference-wise, down the line.
In the future, you should consult with your Job Placement Specialist before taking major steps like quitting a job. Helping you is what s/he's there for, and the proverbial ounce of prevention beats a pound of cure every time.
[*] I say this only because when some people say "I was calm" they mean "I was as calm as anyone could expect under the circumstances" - in other words "The only attitude I gave her was what [I felt] she deserved."
What do you think?