Ken,
His name Johnston as a Indian agent came up only after reading that ,in Jan 1831 Chickasaw agent
Benj. Reynolds,and Choctaw agent George S Gaines meet on a creek on the south fork of the Canadian.
Later named Gaines creek---to look over the boundary for settlement--to the source of Brushy creek-via south Canadian. After a year George S Gaines resigned. Vol 8 pg 315
This became the boundary Chickasaw/Choctaw line--they ended up landing (by boat) Skullyville not this location.
I also think now its a differant Johnson--John Johnston had been a lawyer and Judge,Col.it looks unlikely
to run a trading station/depot at his old age.
Tandy is on the Calif trail near Edwards trading post-unknown name orgin