I propably had the same problem about 14 times with feta, until I also, after cutting the cheese into 3" cubes, placed it in a SATURATED BRINE for 4 hours per 1 pound of cheese, removed it from the brine & placed it on a mesh rack in the fridge to dripdry for 2-3 days, turning it every now and then. This Stabilizes the cheese, without doing this, the chance of having mushy cheese is minimized dramatically. After drying, place in either 8%-10% Brine. Some will say that is too low, but because the cheese was stabilised in the saturated brine, it already has enough salt & doesn't need as much in the brine. When packaging, tru to fill the container up to the lid so there is no air space for molds etc.. to grow.
I used the mushy cheese in dinner dishes as well. Since I did this, it worked wonderful. NB. It is better to use brine from the cheese for the whey as the PH should be fine in it. If you want to, i I'm sure you may add some CaCl as well. Make sure the cheese blocks arent smaller than 3" cubes, or else the cheese will be too hard & salty.
Hope this helps.