I wouldn't do anything that isn't required by the program you're looking to apply to. If they don't require chemistry, don't take it (unless you have the time, money, and desire to do so). Whatever chemistry you need to know for nursing will be incorporated into classes like A&P and microbiology.
Admissions criteria are quite different from program to program, so look into that carefully. Ask an adviser what the average pre-req grades and GPA were of the past few cohorts--this will give you an idea of what to shoot for. For example, my program only looks at grades in A&P I and II, micro, and the HESI. They don't consider any GPA. In the cohort before mine, the average student had two A's and one B in those pre-reqs and at least a 90 on the HESI. This gave me a point of reference from which to gauge my chances of acceptance.
Many schools won't accept online sciences, so I would avoid those. However, I would recommend retaking A&P; not only to raise the grade, but because that information is invaluable and you need to know it well. My program only allows two attempts at the pre-reqs, so find out if your desired program has any sort of limit like that.
If you could provide some more information about the schools you're looking into, someone might have some more specific advice . Good luck to you!