Not sure Jim, We have never let one of our buildings freeze,  Somebody is in them everyday of the week and they will hold heat over the weekend on the coldest days to not freeze up.  I don't have a anitfreeze in any of the three shops we got.  If I am on vaca, I have somebody from the office check my house shop every two days ans make sure she is heating.

Not experience on partially heating a floor, but I don't think you should "pour" the heat too it.  I try and set the water going in the floor at a max of 95 degrees.  If the concrete was frozen that would take a long time to heat up, like days, and then I don't think that would be too fast to crack the concrete.

About testing the tubing, AIR test it before you pour the concrete!!!  I put 40 psi of air in the tubing overnight befoe the pour.  If she hold good to go and it will help hold the tubing from colapsing when it gets stepped on and poured over.  It will get stepped on and I have never had a leak yet, cross my fingers!!! lol