Here's a possibility:
First, there were 4 raincoats - always good to have a spare in case one is damaged or blows away.
Next it was a hurricane that hit card, caused damage - washed away their boat possibly and moved the boulder. Most hurricanes have an eye where the weather is remarkably calm. This may have been the last entry if they set out to make repairs or tried to contact the mainland. They may have suddenly been caught out and too far from shelter when the weather picked up again.
Lastly the conflicting weather reports from the mainland. The only thing I can think is that they lost track of the date on the island, so they were recording logs for the wrong date.
I did think at first that they may have all gone mad, but the logbook reports seemed accurate and sane. That is because lighthouses of this era had glass optics that float on a bath of mercury. Possible some may have got into their food. If the logbook was lost however -or hidden in a fit of madness then anything could have happened after that last entry.