Hello,
I am not a contractor, but I have worked with Durock on my own project. I made a small attempt at getting lime/sand to attach to Durock, but it separated much as you describe. Later, I covered a substantial portion of Durock with a thin coat of gauged dolomitic lime (to level an uneven surface). Despite substantial temperature fluctuations, this has adhered well, though it has been up for only several months. Of course, portland cement also adheres very well. In general, whenever I mix modern and old materials, I have problems. I suspect that the poor adhesion is, as you say, due to differences in expansion/contraction for your surfaces. If guaged putty is suitable base, you might also consider that over well-saturated Durock, but I do not know how well lime/marble dust would adhere to that.