This is clearly an onion waiting to be peeled. Following the "Cherokee Spikes" lead and Livingston connection the biographical information on Absalom Humbard mentions a "Capt Robertson"(Robinson?) of Jasper County. Humbard, Robertson/Robinson, and Rusk consolidated with Livingston to form the 1st MO Cav Battalion. Livingston was a commissioned Confederate officer and his unit was nominally an official unit under the Partisan Ranger act. Humbard, Rusk and Robertson/Robinson remained as Captains in the unit. See: Biographical History of Dallas Co Texas pg 648. Lewis Publishing. Chicago 1892. (Its available on Google Books)

Rusk appears to be Reuben Rusk (served as 2nd Lt Co D, 11th MO Cav, 8th Division, Missouri State Guard) from Carthage, Jasper Co, MO
Livingston we've discussed.
Chenault (see below ) was a Conditional Union man turned confederate.
Then we have this tempting Missouri index card referring to Capt. Robinson in 1861:
http://www.sos.mo.gov/Images/Archives/Military/s00738/s00738_0146.pdf

As you read the history note the story of Humbard meeting a group of families in Arkansas migrating to Texas, sounds familiar. Only conflict is Humbard lists it as fall 1863 vs fall of 1864 for Sarah Robinsons version. Make of it what you may but there appears to be a bit of circumstantial evidence building that William Robinson of Diamond Grove may have participated with Judge John R Chenault's and Sanford Talbot's (Talbot later was Col of 11th Mo Cav 8th Div MSG) Jasper County Minute Men during the MO/Kansas boarder war in 1859 - 1860. Robinson may have taken a number of his men to Livingstons unit in 1861 and participated with that unit until Livingstons death in 1863. He may or may not have been operating independently in 1864 but was "official" enough to respond to Gen'l Watie's order to move to Texas in 1864 ending up at Boggy Depot/ Ft Washita in late 1864 where the unit effectively disbanded.
(Note that the bio history has several misspellings, Chenault/Shinault, Piercy/Percy, Boggy/bogy, so the Robinson/Robertson miscue is plausible IMHO.)

Please note that many of us have family that fought on both sides of the Civil War and by no means should we disparage the fight they fought. Make no mistake, in Missouri for young men between 16 and 60 it was a fight that was kill or be killed without the "honorable gentleman squabble" of Napoleonic warfare that persisted in the East. Missouri was mean, bloody, ruthless, ambushes, small unit guerrilla warfare with depredations on both sides, torture, hangings, murder of prisoners, financial and court corruption. Just plan ugly and folks did what they did to survive, so do not think less of William Robinson, he was a man of his times and circumstance, honor his faith to family, community, and nation as he saw it.

John R