Nothing online I found was clear, but we decided to honeymoon in Puerto Rico and the US Virgin Islands in November 2019. Let's clear this up.
I am a birthright US citizen from Arizona. My wife is K1. We went to the US Virgin Islands and Puerto Rico for a five-week honeymoon in 2019, before she received AP, or Employment Authorization, or anything else. This was about 100 days after her entry to the US on K1.
There were no checks on the flight from Dallas to San Juan. There were no checks in San Juan.
We flew from San Juan to Charlotte Amalie. There were no checks.
Do not go to the British Virgin Islands, even for the day. This is considered leaving the US, and you will not be permitted to re-enter on a K1.
We flew from Charlotte Amalie back to San Juan. There is a formal immigration point in the Charlotte Amalie departure section. They checked her K1, we said we are married and waiting on a green card. They wished us a safe flight with no problems. We flew to San Juan. Arriving in San Juan, immigration agents again checked everyone's passport, including my wife's. They saw the K1, we said she is a green card applicant, they waived us through.
We flew from San Juan back to Houston. There were no checks.
Puerto Rico and the Virgin Islands are incredible honeymoon locations, and it is not worth staying at home after marriage because you read online that your flight could get diverted. The chance of this is miniscule.
If you are K1, make sure you started your 485 and received the Receival Notice, and head to PR and the USVI. It is perfectly legal and allowed.
Have Fun!