The most important swing county in each state

[Donerail]

Florida: I'd say Pinellas-Hillsborough. Both are I-4 corridor counties. St. Pete has heavily Democratic areas & to a lesser extent heavily Republican ones; most precincts are toss-ups. Hillsborough is almost a microcosm of America: Solid D/minority-heavy central city, outlying areas including rural portions that are very R, and tossup suburban areas.

[Donerail]

Also Florida - Duval, which contains Jacksonville. I think whoever wins Jacksonville this time around will win Florida.

Duval is Republican and will continue to go Republican. Duval has gone Democratic twice in the last 52 years, once in 1960 and once in 1976 (and bonus racist points for going for Wallace in 68). Some swing county. Flagler or Osceola County might be a better bellwether as well (or even Pasco/Hernando).

[Donerail]

I calculated this for states that held Senate elections in 2010 by using state PVIs here.

Madison County isn't really a swing or bellwether county, but I'd agree with your analysis for Hillsborough and Flagler. Jefferson might be a bit better than Madison, actually.