Rich, this isn't supposed to be a slanging match, I am just trying to offer advice based on personal experience.
In answer to your questions, in the last 3 years we have taken 4 cruises on Cunard, having carried spirits & wine back on board (from a port of call - not boarding) on 3 occaisions that I can think off, including Gibraltar which is a Duty Free ports. On Princess, we've probably taken 7 or 8 cruises, many of which call into the Duty Free Atlantic Isles, and again everyone always bring drink back on board without incident, it's quite unusual for someone to be returning to the ship without a bag 'clinking' away.
Obviously your experiences have been different, and I wonder if they were on this side of the Pond? Perhaps they clamp down a lot more in US waters.
So as I said, I was just offering a personal insight, and accept that the rules could be enforced at any time at the crew's whim. I hope you can accept my posts as the friendly & informative messages that I always try to provide.
Last Cruise: Minerva April 2015
Next: Royal Princess August 2015
Then: P&O Oriana October 2015
and : Queen Victoria January 2016
Golden, Grand(2), Caribbean, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(11), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(6), QE, Caronia(2), Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey