Since when was Darood part of samaale? I thought it was just hawiye and dir.
This population estimate done by the British during the colonial period, most likely a couple of decades after the turn of the 20th century, has us at 2,530,000. So I think it's safe to say that your estimate of around 2 million at the turn of the century is pretty accurate. So yes, it's truly remarkable how such a small ethnic group managed to conquer the second largest contiguous landmass of any ethnic group in Africa. Also, ur right about us going much deeper into Africa if the Europeans hadn't entered the picture.
Edit: I just realized this population estimate says 'Somali Republic', which means it was done 1960 or just after. This means that at the turn of the century ur lower population estimate of 1.5 million would be a more likely estimate.