Mudster

A substrate for natural stone needs to be 50 % more rigid than it does for porcelain or ceramics.

the 12mm ply would be fine for ceramics, but not enough for stone. Generally I'd recommend aquapanel be glued and screwed over the existing suspended floor to remove localised movement.

There are no circumstances under which I would fit natural stone on a floating floor. This needs to be screeded, no alternative.

If a floor is moving to an extent that it needs a two part flexible adhesive, this is a sign it is not suitable for fitting a natural product.

To add to this, what it would cost to strip and screed the floating area would be saved in substrate preparation and overexpensive and unnecessary products (Ditra Matting and 2 part adhesive). You are then guaranteed no failure of your new flooring.