My only tip would be to look to see if you can arrange your own insurance. Try www.insurance4carhire.com and you can buy the insurance policy here (which will actually cover you for more than what you buy at the desk: tyres, windscreen, etc.)
With regard to the waiving of a one-way fee the only thing I can recommend is booking the best deal that you can get with a company who will let you cancel your booking right up until a couple of days before you are due to travel. Then phone the local office of various car rental companies in NYC and Miami and ask if they have any vehicles that need to be relocated. If they do and offer to waive the fee for you returning the vehicle to its home state then you can cancel your pre-existing booking and hopefully save a few quid. Other than that you're gonna hve to stump up, I'm afraid.