Yes you can change the seats within the F cabin without losing the upgrade.

No lounge access and unless your layover is really long, probably not worth the $$ to buy two day passes.

No free wi-fi (wow that would be a nice perk though)

Row 4 does recline, just not as fully as some of the other rows. Personally I prefer Row 1 or Row 4, as 2/3 feel a bit claustrophobic to me--- still way better than coach, don't get me wrong, but on most domestic F cabins, I go for either the first or last row of coach. Whether or not you find it "really that bad" all depends on how much you want to recline; that said, it's really not a long flight and since you've never been up front before, it's probably not even worth worrying about. It *will* be better than Y, and it's not like the few inches less recline will keep you from getting a good night's sleep-- flight's not really long enough for that much of a nap.

CLT-FLL despite being a hub should generally be an "easy" upgrade even for Silver, or at least easier than most. Florida in general is a good destination for upgrades except during peak cruise ship departure days. High frequency and lots and lots of non-status pax. Much easier than going CLT to PHX/PHL/DCA (hub and focus city routes can be very elite-heavy) or out to the West coast. Factor in a 321 which has more F seats than a 319 or 320, and the non-elite-heavy Holiday travel, an upgrade at the Silver level doesn't surprise me. Although you are certainly welcome to make the sacrifice of your choosing, to appease the upgrade gods, for future flights.