The reason the South Park did not build to Leadville was because of the Tripartite agreement forced on them by Jay Gould. This gave Leadville to the Rio Grande with the South Park having trackage rights. And it gave Gunnison to the South Park with the Rio Grande having trackage rights. Gunnison at the time was expected to become the "Pittsburg" of the west, didn't happen.
They did not use Marshal Pass because Otto Mears had a toll road over it and would not sell to the South Park. After Gould bought the South Park and sold it to the UP Palmer considered the agreement broken and got the rights to Marshall Pass and got his track into Gunnison while the South Park was tied up with Alpine Tunnel. I believe Mears was related to Palmer which is why he sold his route over Marshall Pass to Palmer.
Since the Rio Grand had used the Black Canyon the South Park did start up Ohio Pass but abandoned it when the D&RG connected with the RGW at Grand Junction.
Ken