Elevating the PEX
Yes we "lifted it" - the placing crew was VERY good - they had a "hook" that they used to lift the mesh-pex-rebar "sandwich" and poured concrete around it, let the mesh settle back onto the wet concrete, then let go and poured the rest of the concrete. The pex is approx 2" above the styro base everywhere now.
We put "Zip strips" into the concrete - they are thin plastic strips that function much like a cut through (but without cutting the Pex!) as "crackstops"
Because its indoors, we eventually decided not to use any expansion joints, but I DID leave a 1/2" gap around the edges. I did that by placing a half-inch thick border of styro 6 inches high above the styro "floor" to create a thermal "break" so that when I heat the floor, I am not heating the foundation as well. Ditto the need for a break between the shop floor and the approach slabs - I'm not trying to heat the whole world, and don't need the approach slabs in front of each door to be show and ice-free - I just want a warm floor!
Then when we (OK - my son did this part all himself) sprayed the "cure & seal" on the fresh floor, it dissolved the styro border. After the requisite 28 days, I will then go and caulk that "gap" so that little nuts, bolts, Carb jets etc, etc can't escape.
No drains and it's all dead flat level, set with laser levels, checked on a one-foot square grid as the placers and finishers worked.