I would not suggest using the thin-set mortar for the simple reason that it does not react well to winter freezing and will crack with just a little bit of movement. If you do want to change the slope of the deck, perhaps the best way is to install tapered layers of rigid foam insulation over which you can put a waterproof membrane. It is a tedious job, but it will work. The other possibility is that if all the water is concentrating in the center of the deck, perhaps installing a drain there will allow you to avoid the tedious task of correcting the slope. The problem will be finding a route for the drain pipe without going through the middle of the room below. If the drain pipe enters the heated portion of the house, cover it thoroughly with insulation and a well-sealed vapor barrier.