I recommend skipping the cruise tour. Instead go a week or 10 days early, rent a car and make up your own tour. You see what you want for as long as you want and are not tied to some schedule. Also suggest using the train for transport. Then when you are done on land book a one way southbound cruise from Seward to Vancouver.
To a large extent it depends on where you are. I places like Santorini they are likely not going to allow friends to join because the process is so slow.
Switching between MIA and FLL is no big deal. Just get off first ship, grab a cab or Uber and 45 minutes later you are at checkin for the second ship. Don’t over think it. You have about 5 hours to go 20 miles.