The cabin floor of all the cabins appears to be above the 100-year Base Flood Elevation (BFE). The roadway along the 3 cabins closest to the river was slightly below the BFE, so those cabins were evacuated first.
http://pxl.to/Mystic-FEMA-flood-map-analysis
Historically, the flash flood warnings have rarely resulted in any flooding of significance and many of those warnings included the same "life threatening" language.
http://pxl.to/Mystic-NWS-Flash-Flood-Warning-Analysis
When considering the information from above, combined with the weather forecast that did not predict anywhere near a 500-year rain event, it seems reasonable to use a phased cabin evacuation approach. The only information they had to act on was visually observing the water level of the river. The combination of the rapid rate of water rise and total amount of rise made evacuation difficult. There likely was not enough time to change plans by the time they realized how serious the flooding was. Here is an estimated timelapse video in 10 minute increments:
http://pxl.to/Mystic-flood-timelapse-video
https://pxl.to/mystic-4d-timeline
I think the phased evacuation might have been successful in either of these scenarios:
Rapid rate of rise but max flood height only reaching somewhere between the 100-year and 500-year flood elevation. The ground outside Twins/Bubble was about 3ft above BFE and the floor was another 2ft higher. These cabins would be the last to flood and would be safe for flood heights up to 7ish feet above BFE (the July 4 flood was about 11 feet above BFE).
30ft rise but slower rate of rise. That would give them time to make it safely to the Rec Hall where the 2nd floor was above the 500-year flood elevation.
Unfortunately, the river rose extremely rapidly and higher than it ever had before with little warning.
Here is a overall big picture analysis of the flood event.
https://pxl.to/mystic-analysis