Ok Rob, Let me take a stab at it.
There are several medical units in the field in Natal which leads to the confusion. Besides the NVIAC there are the Dhooly Bearers from India & the Indian Hospital Corps.
To my mind Dhooly Bearers carry dhoolies & stretcher bearers carry stretchers. To add to the confusion there is also "Messrs C.Dhanji Bhai and Company's Tonga Train" which used Tongas but not in Natal. So going through your photos, The first I would say was a Tonga, the third a stretcher, & the 4th a Dhooly.
As for uniforms, I attach a picture. The dark tunics I worked out from the numbers on the rolls as being the Indian Hospital Corps. The Dhoolie Bearers were issued kit so more likely to be wearing uniform, or parts of it. The Indians, volunteered from the farms & mines were more likely a motley crew. The slouch hats of your first photo are similar to the Boer ones - I agree this is more likely to be the other 1906 unit ( BTW there is no continuity.between 1899 & 1906 other than Gandhi since he was the only one to serve in both on the rolls) since the "team" photo of the 1899 photo has Booth seated in the middle.
As far as casulties, I have found none to the Natal Indians but I can offer you these,
608 Dooly Bearer Gopal Pari of the Dhooly Bearers Mhow District served at Belfast, Elandslaagte, Ladysmith and Laing’s Nek. He received a gratuity of Rs64 for wounds sustained.
L/AG/26/11/2 (Folio 41 South Africa 36080/02)
WO 100/298 p145
142 Dooly Bearer Karati Paupiah Dead
WO 100/298 p162
242 Dhooly Bearer Yatoor Kondiah served with the Madras Dhooly Bearer Corps at Belfast; Talana; Defence of Ladysmith and Laing’s Nek. His widow Etoor Aukammah received a gratuity of Rs70.
L/AG/26/11/2
WO 100/298 p160-2
373 Dooly Bearer P.Yelliah Bellary District Deceased WO 100/298 p167
M.M.25 Dooly Bearer Binda Punjab Command-Meean Meer District Dead (Ladysmith) WO 100/298 p168
M.M.57 Dooly Bearer Ram Din from Meena Meer died Ladysmith. His mother, Sidhai was granted a gratuity of Rs 50 from 1st February 1901 (Pen Cir 141) Punjab. Caste Kahar
L/AG/26/11/2
WO 100/298 p170
M.M.27 Dooly Bearer Pitai Wounds
455 Havildar Munesar Singh served with the Dhobi Corps in Natal (attached from the 8th Rajput’s Bengal Infantry as paymaster) He shot himself on the 13th March 1902. Singh was buried at Howick. His name is absent from the memorial erected there. Queen’s Medal issued 18th January, 1905 and King’s Medal 20th January.
I have no evidence that NVIAC were at Spioenkop or Tabanyama although there were members of the Indian Mule Corps transporting water to the summit of Spioenkop.
Make of it what you can. Let me know if I can be more useful
ATB
David