A typical bathtub holds around 40 gallons, which equates to 8 pints of water, and since a pints a pound the world around, that means 320 pounds of water+you in the tub plus whatever part of you is above the waterline.
If we estimate the surface area of the bottom of the tub at approximately 4' by 1.5' that's about 6 square feet or 864 square inches.
That means that at most the pounds per square inch on the bottom of the tub will be about .4 - and that's ignoring that part of the weight will be carried by the sides of the tub around to the rim.
Then there's the mortar. 1 cubic inch of mortar comes in at about .08 pounds and 3 of those vertically means .24 pounds bringing the total pounds per square inch on the foam, maximum, to about .64.
So the concern here is - will the foam take the weight - and the only way to find out is to determine if the foam is able to withstand pressure of .64 pounds per square inch without compressing. And also - will the foam's strength deteriorate over time.
These are answers you'll have to find out from the manufacturer. (Although you can test it yourself - use weigh the head of a hammer resting on a scale while the handle rests off the scale and that is probably > 1 pound... then rest it on the foam) But if the foam is rigid enough to take that kind of weight, you'll probably be ok.
Any shower I have done in the past 5 years has been done with both the PVC liner over a sloped bed and the liquid applied membrane. I coat all the corners and the drain 1 coat, then paint 2 coats over everything up to the splash line of about 5 1/2 feet. No need to do the ceiling.
If you have not done it already, let me suggest this.... I use a modified thin set and fiberglass tape to cover all the joints before applying the liquid membrane. It strengthens the joints where the cement boards meet. Also bridges gaps for the membrane.
Best Answer
The existing drain assembly terminates at the height of the current shower pan. Your proposal would require a drain extension to the new, higher shower pan. This would require a smaller diameter fitting, inside the old drain, in order to extend the height of the new drain. This would be practically difficult, if at all possible, because of the short lengths of pipe involved unless you're creating a "step-up" shower. By the way, such a fitting violates code by reducing the diameter of the waste pipe (even though just at the shower pan).
To avoid the above, the old shower pan would have to be excavated, at the drain, to allow the old drain assembly to be removed and a larger fitting attached; into which the new drain would be fitted.
Note: raising the height of the shower pan will result in faucet handles and shower heads being lower than they are now.