I can tell you what I did, that seems to work
First of all, cut the micore with a knife, not a saw, you do not want to breathe in the fibers. The docs at efireplace say you can mortar the tile directly to the micore ( I did not)
I lay down a layer of aluminum flashing to even out heat that gets throught the insulating layer. This was followed by a 1/2 in of micore and 1/2 in of durock next gen. The micore was secured to the underlayment with short durock screws , 8 in on center. The durock was secured to the underlayment with long durock screws, 4 inch on center, that reached all the way to the plywood subfloor. No glue was used between
My tile guy just laid the pieces next to one another, making sure the seams on one layer did not overlap the seams below. The efireplace site says to use furnace cement in the seams of the micore (I did not) . The tile places make a big deal of taping the durock so as to avoid cracks ( this is more a problem with walls). The tile guy tells me he laid down a skin coat on the durock.
Taping a hearth floor, is, in part , a method to avoid hot embers falling through cracks in the tile/fiber/cement board onto the wood below. If you've got a metal sheet on the bottom, this takes care of the 'ember fall through" problem
Finally, to add strength to the mortar layer he used 1/4' hardware cloth (metal screen). The mortar and grout itself was called flexbond an polyblend, which has polymers that make it hold a bit better under bending. If your grout lines are 1/4"in or more, you want to uses sanded grout for strength
You can lay the micore directly on the floor. In my case the sheets were edged on the side by the floor and subfloor. If you lay it on top of an existing floor, I'd use a wood trim to keep the edges from fraying
ref
https://www.hearth.com/talk/threads/mineral-board-or-micore-sources.87737/#post-1404860
