There was one error I think is important to point out. You wrote:
“The only building with a recent lick of paint, however, is the Ithna sheri Dispensary, built as a dispensary for the poor by an Indian merchant, Sir Tharia Topan, in honour of Queen Victorias golden jubilee.”

The error is that the building was not originally the Itna Ashari Dispensary, as Sir Tharia Topan was not an Itna Ashari. Rather, He was an Ismaili (hence the link to the Aga Khan, who is the currently leader of the Ismaili community). Many people mistakenly refer to it as the old Itna Ashari Dispensay because after Sir Topan died, his widow sold the building to an Itna Ashari fellow, who then operated it for a number of years. The building was actually originally called the “Tharia Topan Jubilee Hospital”.

Some background on the history of Zanzibar and this specific building can be found at the AKDN web-site at:
http://www.akdn.org/hcsp/zanzibarprojects.pdf