It is a shame that so many people are being detained without any reason. Unfortunately, the port authority can detain you long enough so that you could miss the cruise, so this is used to their advantage.
Guests who are not Priority are allowed to board with their Priority cabin mate(s). That has been permissible for years and is not "sneaking" on or otherwise breaking any of Carnival's rules.
When we cruised and our daughter was young (about 7 years old), we found late dinner to be preferable. She could run around most of the day, take a nap about 4 or 5, and then be ready for dinner at 8.
I concur. Allows you to check-in from anywhere instead of making 2 trips to the maître D's desk. The app also gives you an estimate of how long the wait is.