This is a very detailed question with tons to answer.
I ask builders to side on a larger recess than needed. Preparing a negative form of about 3.5" will give you loads of options. smaller negative voids are possible but you need much more precise planning. At concrete placing time you do not want to be building a piano. A simple 2"x4" form will do the trick. We ask they extend out past the shower's wet zone.
Before doing any math or form placement you should understand the requirements for tiling over a radiant floor with heat. Get the entire assembly worked out.
Do not plan to heat the floor under the tub or in the new shower with the radiant pipe if possible. We have not done this and prefer using cable heat.