Reneging after verbal acceptance at retail bank

Hey,

I've received an offer for a summer internship at a major retail bank, they want me to verbally accept tomorrow. However I still have other applications in process, ones which I would much prefer, if given a straight choice. I have a couple of questions for anyone who has been in a similar situation:

1. If I were to accept and then reject the retail bank (i.e. I get a better offer) will I be blacklisted from said retail bank? Do companies share this sort of information with each other?
2. How should I go about a potential rejection, phone or email?
3. Apparently location isn't decided until April, could I demand to be in a preferable
location/department within my role?

First question is the most important Thanks for all your help!