I appear to have, eventually, found out how to resolve my issue but was not satisfied with AirBnB's support nor could I find this documented in their Help Center nor elsewhere on the internet.
I include the high level explanation here and a link to the details of my Support engagement with AirBnB . . . they never confirmed that they would spread this information across their support teams or create a Help Center article to address this, so I am sharing it here with the hopes that other owners/managers can answer this question without the struggle I experienced.
Problem:
4 hours for a same day turnover (Check-out 11 am, Check-in 3 pm) wasn’t enough time to complete a same day turnover for my property, so I changed the Check-in time to 4 pm.
Quick Replies that I set to use the Check-In Time variable were not reflecting the new check-in time, so I contacted my guests.
Solution:
Hard code the time in messages for existing reservations. New reservations that are booked after the change in the AirBnB setting will reflect the new value.
Tl;dr
As the check in time appears to be saved at the time of the booking, then making this change does not populate the Quick Replies automatically. Updating the Quick Reply to show the current check in time will insure that messages will show the current check in time, but should this variable be changed in the future, the messages would need to be updated as well.
**[Link removed as per the Community Center Guidelines]