Thanks for that Chris,
It is pretty clear from the all the information dug up in the last two days, ie. paper you found, Andrew's observations, the Balazs paper and Bevington's diary, that the turtle remains don't tell us anything about the time of year the castaway made camp. And to top it off, Dr. Balasz, the NOAA turtle expert, very kindly replied to my email today and his opinion is likewise. According to Dr. Balazs, juvenile turtles are typically present year round on islands such as Nikumaroro (as Andrew surmised--a turtle nursery in the lagoon...), and while breeding is seasonal, the breeding season is a long one (he said for many summer months).
Oh well, it was definitely worth nailing this down.