Ryan,
You ask a very interesting question.
I had initially thought the SAMIF was a term used to refer to the mounted units raised during the war and that it excluded pre-existing units such as the CMR. It may well have been an Imperial short hand to encompass the many units that were raised.
As you note, there is a medal roll for them in WO100/276:
276 South African Mounted Irregular Forces, Standerton Mounted Police, Steinaecker's Horse, Struben's Scouts, Tembuland Mounted Rifles
They were recognised as an official unit as this note from 1913 acknowledges.
The medal roll runs from WO100/276p2 to p84.
As you say there are medals named to the unit. For example: Lieut. H. Jowers, S. A. M. I. F.
Some of the roll pages are titled SAMIF Staff so I wonder if they the SAMIF had at least a partially administrative roll for the irregular units?
So, my answer to your question is that the SAMIF is both its own unit and an umbrella terms for the locally raised mounted forces in SA.