First of all, I'm sorry you've had such a bad experience with the education system.
But good on you for not letting that stop you and deciding to return.
Definitely.
This is coming from someone who sat her GCSEs last year at nineteen (I left school before I completed them). I don't know what your finance situation is, but the only problem might be -- because you've already gained GCSEs -- I don't think you'll be entitled to funding for it.
I wouldn't think so.
Universities can acknowledge a person will have changed a lot in the years between sixteen and nineteen. Also, I imagine what you experienced could count for extenuating circumstances, in which case you could get your referee to explain what happened when you originally sat your GCSEs in your reference so the universities are aware it wasn't a true reflection of your ability.
There used to be five GCSE bundles for adult students -- they've been swapped for Pre-Access courses at my college, although I don't know for certain whether it's the same nationwide. Even still, you can pay to do individual GCSEs, yes.
On that note -- have you considered an Access course? Most Access courses only require English Language and Maths at grade C (some of the ones that would lead you to Nursing may require Biology and most universities require a grade C GCSE in Biology to do Nursing, too, I do believe, although don't quote me on it).
You can't do A Level bundles at college as an adult student, I don't think, no.
You could pay to do individual A Levels, though, but like I said, I find Access is a much more convenient option for 19+. Also, if you've never done a level three qualification before, you should be entitled to government funding.
I'd recommend making an appointment at your local college, to be honest.
They can advise you on the best plan of action.
Best of luck!