Setting different shaders in blender2crystal seems to have absolutely no effect however I take it there is a way to fix this.
The effect setting a shader has depends on some additional parameters; specifically, which rendermanager (or renderloop in case of using "legacy" render loop rendering). Might be best to try to find someone with practical experience to give you a rundown ...
Would I be right in thinking that if the engine can do the calculations for a reflection it can apply a texture as well, so that the UV mapped texture assigned to the mesh provides a base colour (similar to the effect of setting up ray mirror in blender)
Certainly. It's all a matter of what the shader does.