Jay,
As to the transit time, before the Usambara Railroad reached Moschi (1911, IIRC), the transit time depended on the progress of the railroad construction from Tanga to Moschi. Before the railroad, the mail was carried by messengers on foot the full distance to Tanga (~350 km). During that period, 10 days was typically the minimum transit time. As the railroad was slowly expanded toward Moschi, the transit times could be shorter, since the mail theoretically only had to be carried as far as the western terminus of the railway. However, even with the railway partially constructed by 1903, transit times could still be slow, as a large portion of the route was still on foot, and the train service was largely dedicated to construction needs rather than regular passenger and mail service until the line was completed and formally inaugurated in 1912.
My primary source for this info is the new book by Dr. Soll that Oliver mentioned in the Show and Tell – Die Postämter und Postagenturen in Deutsch-Ostafrika.
As to the lack of an arrival cancel, no idea. Could have been missed somehow, or it never made it there.

