Is the ending (terminal?) pH of a make predetermined by part of the process earlier in the make?
Yes. Think about acid development as a function of bacteria. The bacteria keep eating and multiplying so long as the conditions are favorable. Meaning temperature is good, there's enough food, and nothing has triggered the bacteria to die (such as overpopulation or high toxicity). Species and strains differ in their needs and tolerances. For example, bulgaricus tolerates acid really well, down to below 4.0. Whereas some strains of Streptococcus slow down around 5.2 and don't go below 4.8 at all.
buffering by calcium and other salts also plays a part, but not as much as temp and food and conditions and strains.
For example the pH at draining,
When you drain, you remove food and freeze the bacteria in the curd. Makes it harder for them to make acid. They have to work more. When you drain, typically, the curd cools down, which also tends to slow down acid.
or the pH at milling/salting,
Salt slows down acid development, and by this time temp is actually at least 10F lower
or the pH at brining.
Similar to salting
Will it not drop any further than that pre-determined point?
Depends on colony health, number of bacteria, temp, strains, and conditions.