Going via DFW will have you flying past Florida and then back. Not a big deal if the times suit. Just make sure you won't have to change terminals - DFW is huge.

Have you considered flying into TPA? You don't say where in south Flroida but TPA is a much nicer and easier airport than MIA by a long shot.

You will have to make a connection to get to RSW unless you fly into TPA or MIA and then drive. If you take this choice definitly fly into TPA!

Flying on Christmas day will be dead quiet. Have done it and every place is empty.

Transiting through any of those airports shouldn't be to bad weatherwise. PHL and CLT may be the only one with cold weather issues.

My choice would be LGW - TPA nonstop then just drive to wherever you want to end up.