Nevermind. With further digging, I think I've answered all or most of my questions.

I have a 3/4" maple floor over a 3/4" plywood subfloor on the second story of my Chicago home. I would very much like to install a solid granite hearthstone in front of my gas-ignited woodburning 36" fireplace and have it be flush with the floor, but I'm having difficulty thinking of a way to do so that wont have major risk of the granite cracking. I plan to defintiely use Ditra to uncouple the expansion of the floor from the expansion of the hearth stone. However, I can only use 2cm, not 3cm in this space, and I can't fit any Hardiebacker / Durock / etc. nor any mortar depth without pushing the 2cm stone above the surface of the floor. I'm assuming it would be a very bad idea to cut out the subfloor and try to attach a piece of supporting wood beneath it, since then I'd have granite hanging by screws. There's not much general foot traffic on the hearth, but there is a cabinet above it, and sometimes I stand on it when moving fireplace logs, so I don't think I can assume no one will walk on it. The dimensions of the stone will be about 55" wide by 16" deep. The firebox opening is a little over 8" up from the floor. Do others think I can get away with installing just Ditra and thinset and 2cm granite directly over plywood? Also, if I use Ditra, does anyone know how much thickness that will take up? And what is the thinnest layer of thinset I can get away with to bed the Ditra and the granite? Also, is there a particular type of thinset / mortar that would be best for this? Thanks!

I know , but constructive criticism only please . How would a pro fix this if it is incorrect?

What is the nature of the failure on the plywood you're referring to? I've probably laid hundreds of thousands of square feet of plywood over concrete and can't think of one failure I could attribute to the installation method (i.e. the failures have all been caused by other factors, like a flood). Granted we go a little overboard in that we flat trowel cold bond adhesive on the concrete at 50'/gallon, lay in heavy poly sheeting, trowel another layer of mastic over that at 32'/gallon, lay in the plywood, shoot it down with 1-1/2" hardened coil nails and fasten any loose areas with tapcons. Before sheet vinyl fell out of favor we used it under plywood all the time in conjunction with adhesive. I've had a few jobs recently where the client wanted something better that visqueen so we used a membrane like Traxxshield99 with seam tape. The plywood subfloors I have seen fail due to installation method, are almost always laid dry over a single layer of black felt paper (which is not a suitable moisture barrier), then shot or screwed down. I don't think there's any need to build up a subfloor like that and affect the relationship to doors and steps, but if the advantages were deemed to be beneficial, I'd recommend a moisture barrier, 1x4s fastened to the concrete 12" on center and a layer of 3/4" plywood over that, with the joints laid on the 1x4s. This can be a little complicated because it would be nice to have p.t. lumber for the 1x4s but they're generally soaking wet upon purchase and there may be an issue using steel fasteners.