There has always been a two shot requirement for anyone not eligible for the J&J vaccine. Nothing has changed regarding testing and vaccination since February.
Well not specifically that app. But it's a popular 3rd party chat app that isn't tied to your cell phone carrier (and thus may not work all the time or at all on board ship)
I'm going to go out on a limb and guess that there are other things in the world besides covid that cause people to cough.
And the crew masks because they have to, not because they want to.
You can use your carrier's messaging if you have wifi calling enabled BUT it's no guarantee to actually work.
You're better off with a 3rd party tool like Messenger or WhatsApp
I live at high altitude, so going to sea level I always get congestion and sore throats in the morning. But in the back of my head it's "well what if it's covid and someone rats me out for coughing"
It's more guaranteed seating than reserved seating.
And you don't need to ask the concierge about kid food stuff like that. CK (or MDR) can do that on demand since there is a kids menu.