Airlines almost never reimburse for rental cars.

You might be able to get the value of the DTW-BUF segment refunded (or at least moved to a voucher you can use toward a future flight). But you'll probably only get that if the delay was NW's fault. Medical emergencies are almost never the airline's fault, and a late incoming plane can be caused by all kinds of things beyond the airline's control (weather, ATC, etc.).

Given that you already got a valuable $75-off-$300 flight, and they offered to pay for your hotel for the night, and the delay was only 2 hours (hardly eggregious), and there was a medical emergency involved, you don't have a whole lot of room to negotiate. But a rationally worded letter to NW requesting a refund of the DTW-BUF portion of your ticket can't hurt (the worst they can do is say no).

Note also that if this was the outbound portion of a roundtrip ticket (i.e., you'll be going back to FLL in a couple days), you'll probably find that your return was canceled when you failed to show up for the morning DTW-BUF filght. You ought to call right now and try to straighten that out if you want to be able to go back to FLL without buying a new ticket. I'd make that my first priority before worrying about compensation for a 2hr delay.