Very, very little heat comes off the back because the stoves fire box rear is insulated with brick. And, their is the extra layer of metal covering on the stove back that traps air which is forced up by the fan. The sides of the stove are also insulated by firebricks. The sides do get hot though and I redirect that heat into the room with a piece of sheet metal on each side of the stove. Probably not mush gained with that to be honest. My stove is about 1/3 inside the fire box, 2/3 on the hearth, and the fireplace opening is 4 ft wide. Most of the heat is from the top, front window and with blower on.
Block off plate is must do.
Your idea of lining the inside the the fireplace is good mostly if the fireplace has walls that are exterior. Meaning, the fireplace walls are not in the middle of the house or surrounded by rooms.
Research inserts if you have not. When you mention insulating and durock the inside of the fireplace, makes me think insert. Again, if your fireplace wall(s) are exterior, it may be worth doing no matter what type of stove you install.
My fireplace walls are interior and I dont see a reason to insulate. In fact, the block off plate where the liner goes through it, then up into the chimney, gets hot. And the fireplace firebox is a little warm. So, maybe some of that heat goes back into the house or heats up the masonry a little and slowly retains and radiates heat into the house.
Every house is different. This is what works in my house with my stove install.