Here is a modified version of my TIMELINE from post #438, slightly different from CHATEAU's due to my assumption that the puzzle place closed for 6 hours.
1. Dhows leave DAR dock at 530am (C/M, O/D), 830am (U/J, D/K), around 530pm (E/D) and 830pm (I/T, J/B), but puzzle place was not open after midnight(assume it opened 6am).
2. Crossings take 6 (I/T) to 9 (D/K) hours. Some may have taken more, but there is no way to know which ones. I will use 6 except for D/K and U/J
3. From west coast Zanzibar island to the museum was supposed to be 1/3 mile on foot, 5 minutes. Logs were 2 miles by taxi, then 1 mile by cart.
4. Puzzle looked like it could be done in less than 20 minutes by a sharp team(U/J or J/B) or 30 minutes for a slow one like Ian/Teri.
5. Logs could be done in about 40 minutes to get to the source, load, travel, and unload.
6. Distance to rungu toss was 15 miles or 30 minutes.
7. Rungu toss looked like to took 10 to 30 minutes.
8. Trip back to Old Fort, Stonetown would probably be in the 30 minute range.
Add it up and you get:
C/M finish line at 1400.
O/D finish line at 1410.
U/J finish line at 1945.
D/K finish line at 2015.
E/D finish line at 0125.
J/B finish line at 0740.
I/T finish line at 0745.
With this detail, I get 17 hours, 40 minutes as the lead Charla/Mirna have over Joe/Bill. The key to this is the assumption that the museum closed for 6 hours. This was substantiated by the fact that they were in daylight when exiting the museum, which if I/T and J/B landed Zanzibar around 0230 would have been impossible.