I think it's always important when looking at any of these post-loss messages to think not just phoenetically but in terms of people tending to translate unknown information in terms that fits into their realm of experience. I.e., if someone said the uncommon word "detritus" most people would not immediately understand it, and would first try to fit it into their own experience (e.g. "Detroit what?").
I've always thought fig.8 was 58 -- another occurrence of the 158/338 fragments which seem to keep repeating throughout the notebook.
Back to the topic at hand...this seeming discrepancy, rather than calling things into serious doubt, seems to me to have potential to give us some serious insight as to what happened.
I know that the current theory is the plane got dragged over the reef. Is there any reason to assume that this happened all in one day? Consider, if you will, the possibility that the plane was being dragged a measurable distance towards the reef flat by each receding tide. If that's possible, then consider the possibility that by the 5th, the plane had gotten so far into the deep water that Earhart started to fear that she was about to lose the plane and that each potential transmission might be her last. It also contributes mightily to understanding the panicked atmosphere within the plane and the obsession with the water level. Betty's Notebook, which I tend to believe, did always strike me as being a bit over the top....HOWEVER, if the situation is the plane is getting closer and closer to the reef edge every day and into deeper water, and there's a very real possibility that this transmission might be their last, it explains both the panicked atmosphere and why this particular transmission went on so very long....they might have seen it as their very last chance to transmit.
It also dovetails with what little we know of what happened next. After July 5, little to no radio activity. By July 9, there (apparently) is no visible plane and, (again apparently, we don't know for sure) no sign of Earhart and Noonan on the beach, either. If the plane goes over July 5-6, that gives our heroes 3-4 days to assess their situation, fail to be rescued, and then to possibly decamp.
It all tracks to me...better, in fact, than it has done up 'til this point.