Knobbygb - go for it!! I've only ever driven around Everglades - the idea of 100's of miles driving over lots of bridges / islands never really appealed, so I never really went down to Key West.
IMHO, lifejacket would be strongly advised.
To answer your 1 - 4:
1. Yes
2. Without looking at a map, I'd tend to opt for the 'along the road' route on the basis that if you do have to ditch, someone might see you and assist / raise the alarm.
3. I'd opt for a lifejacket.
4. No man-eating sharks, but there are plenty of crocodiles in Florida! Before the pedants get me, I believe that crocs are only an issue in fresh water, but you never know - some of them may have got a taste for food with salt on!!
Also, having done sonar trials for several months off Andros Island in the Bahamas and flown in US Navy aircraft out of West Palm, be warned - the sea isn't
that warm - only a few degrees warmer than the Atlantic. Survival times don't suddenly become days!!