I suggest you don't turn off "Consolidate Materials". This option was included for very specific cases, but is rarely used.
Forest creates a multi-sub with all materials defined in the Geometry List. This is necessary to handle multiple objects with different materials (some of them would use multi-sub as well). You can find a detailed description in the documentation
As general rule, just assign your materials in Geometry->Material
and don't worry about the forest automaterial (this is generated automatically). When "Material "parameter is not defined, Forest takes the material directly from the source object.
Regarding VRayMultiSubTex, is not supported within a Forest object. Instead, we suggest to use Forest Color
. A discussion about this issue can be found here