Ah, got it. Then the cause is twofold. One, your proteins were denatured too much from the heat, and two, the acidity was too high.
Ricky is wrong.. maybe that's too strong... I should say I prefer a more conventional cheddar approach to hers... you don't put it in at 50f as soon as it's out of the press. Cheese is a living thing, it needs a gradual adjustment, and also a day or three to air dry at a moderate humidity level of 70% or so, and room temp or a little cooler. Air drying for a few days will not make your cheese dry. By this time, there's little you can do -- the cheese is made in the vat.
I'm a little lost on the 12x multiplier. A multiplier is not something you derive. It's a fixed constant. Flocculation is the point of when the milk gels over, when it tips from a liquid to a gel. The multiplier is fixed to give you a total time from the point you add rennet to the point when you cut the curd. So if it takes 15 minutes for the milk to gel from the point you add rennet, you take that 15 mins, and for cheddar a 3x multiplier is good, and you get 45 minutes total that you need to wait from the point you added rennet to the point you cut the curd.
Your latest make with at 15 min floc and stopping when the cheese was squeaky sounds just about perfect. I can't see the cheese, but if the curds taste good, the cheese should as well.
The ideal time to add rennet after adding culture depends on what you're doing. If you use a bulk starter at 1.5-2%, you can add rennet pretty much right away. If you use DVI culture, you need to wait 30-45 mins for the culture to wake up. When it wakes up, there should be a very slight drop of .01-.1. It's not about a specific pH, but a slight drop in pH.
We had a discussion of cheddar pH values before already, but to recap, rennet at 6.5 (this is about what fresh milk is, higher is OK, a bit lower is OK, too), drain at 6.05-6.25. Lower will give you crumblier cheddar. Cheddar until 5.4, then mill and salt and press.