1. We're regular riders on NJ roads at 16-18mph. Should we just plan to stay on the highway? Trail seems tempting, but might be "suboptimal"
This seems like a subjective, personal choice. You can get a really good idea of the road view by using google maps and dragging the little yellow person icon down onto the road for the driver's view of the road at street level. As for road conditions, I read previously that with the hurricane cleanup, debris was pushed to the side of the road initially. It might be cleaned up by now, but there could be small nails/glass? Might be best to ask a local bike shop down there about that issue. I'd bet that the trails and real bike lanes might have been cleaned better. Another issue with riding on the road itself is the traffic, which is frequently heavy. When are you going?
2. Probability of rain? should we even care if it's warm?
When are you going? If not in the hurricane season, then probably a chance of passing shower; shouldn't care.
3. How many tubes would you take? Think flats are more likely than what we typically see up here in the Landfill of Opportunity?
How good are you at roadside patching? I took two spares on my last ride through the keys, and had two flats. One was patchable, one was the stem (old tube, or maybe my fault).
For a good laugh, watch this series of videos. youtube.com/watch?v=BKSQjXmB0q8
4. Any particularly bad spots on this trip?
I suppose the Seven Mile Bridge on a windy day could be treacherous. Otherwise, there are only a few miles of narrow shoulder riding. IMO, there are no other bad spots.
5. Any suggested "stop for food / stop for a memorable malted beverage here" places?
Florida Keys Brewing Company. Friendly people. Outstanding beer.
6. IF we should have some "massive fail" - who would ya call? Uber? AAA? J.Buffett?
I ride a Brompton, so I would just call any old Uber (and have done that down there). People are super friendly, so I'd bet a local would give you a ride to a city center. You might also get a list of local bike shops and have their phone numbers handy.
And if you were joking, definitely call J. Buffett. Or Peter Anderson.
I should be doing a Miami-Key West trip again in a few weeks, so if I learn anything different about road conditions and debris, I'll share here.