I don't have tons of experience on Carnival, but based on past experience and on a recent carnival cruise, I would say dress like you are going to a sit-down family restnt at home.
Seems that many cruise lines are now allowing a self administered (not proctored) test. Except for those itineraries with ports-call that have more stringent requirements.
Curious. Why would a phone not work in wifi calling mode when off the ship? We use wifi mode all the time. All we need is wifi. However, we don't use ATT. Maybe that is the difference.
You do not factor in that a longer period of exposure might cause more spread among all those people. One hour among 3,000 people is not the same as 2 weeks among those same 3,000 people.